Severus Snape is both a member of the Order of the Phoenix and a Death Eater. He was Harry Potter's least favorite teacher at Hogwarts. He was the Head of Slytherin House before becoming Headmaster in 1997 until he was murdered by Lord Voldemort less than a year later.
Severus Snape grew up in a dismal looking industrial town in a lower class area called Spinner's End. He seems to have had a fairly miserable childhood with his parents arguing with each other all the time. In a flashback scene from his memories seen in the Deathly Hallows we see him as having been dressed in an unfashionable and shabby coat that made young Petunia Evans mock him infront of her sister, Lily, who Snape already seems to have had feelings for. This prompts him to retaliate perhaps unintentionally (him being too young to properly control his magic) by cracking a branch over her head by magic. As a child, he grows fond of Lily Evans (later Potter), because of her kindness. They both enter Hogwarts in the same year, Lily goes to Griffyndor House and Severus goes to Slytherin. They grew apart because of eachother's friends and behaviours but Severus never stopped caring for Lily. That is why he needs to protect Lily's son, Harry, when he arrives to Hogwarts. Severus has mixed feelings, since Harry is the son of his beloved Lily and also the son of James Potter, one of the bullies who used to mess with him at school. The feeling Severus had for Lily were so strong that even his patronus was a doe just as Lily's.
Severus, although seemed otherwise, was very loyal. At least Albus Dumbledore and the Dark Lord thought so.
Harry first notices Snape when dining in the Great Hall on his first evening at Hogwarts. He was startled by the look that this unknown teacher gave him that he later thought was dislike. Even more disturbing was a sudden pain he felt in his scar at the exact time Snape looked over at him. Snape was Harry's Potions Master, who seemed to show overt favouritism towards members of his House, Slytherin and something close to hatred for Harry.
Harry believes that Snape is responsible for the mysterious goings on at Hogwarts and that Snape is trying to kill him and retrieve the Philosopher's Stone. As it turns out, Snape was grudgingly trying to save Harry's life, though he did detest him because of a childhood grudge against Harry's father.
Severus Snape makes an unexpected first appearance in this book by surprising both Harry and Ron partway through a discussion about whether or not he'd been sacked. He quickly brings them to his office to punish them for arriving to Howarts not by train but by a flying Ford Anglia risking the exposure of their kind to the Muggles.
Perhaps one of Snape's most significant contributions during the series occurs in the second book when he teaches Harry and his classmates the "Expelliarmus" charm during a Duelling Club meeting. This becomes Harry's trademark move, and saved his life while dueling Voldemort in Goblet of Fire, and at the end of the final battle in Deathly Hallows.
If Snape had been annoyed and angry with the person that got the post of Defense Against the Dark Arts teacher ahead of him last year, it was nothing to his anger at the choice this time. Albus Dumbledore has this year passed his application for his old classmate who associated with the bullies of his youth and was the source of his ignored accusations of being a werewolf, Remus Lupin. Snape eventually caugt Sirius Black before being knocked out by Harry, Ron, and Hermione. He later accused them of being confunded.
During the first book, Voldemort is gradually raising power. Also, this is the first time we see that Severus Snape was indeed a Death Eater, though he turned spy for the good side,and we realize that it was the Dark Mark gradually becoming darker on his arm that was frightening Karkaroff. In the end of the book, Dumbledore has him reestablish his role as a spy. Snape and " Moody" (really, Moody's imposter, Barty Crouch Jr.) had an enmity.
Snape is a very important member of the Order of the Phoenix. His main job is to spy on Voldemort and the Death Eaters, a job that Dumbledore says he would entrust to nobody but Snape.  Snape attends and gives reports at Order meetings. When Dumbledore becomes concerned that Voldemort might try to read Harry's mind, he asks Snape to give Harry private Occlumency lessons. Because of the strong animosity between Snape and Harry, those Occlumency lessons do not go well. The lessons end abruptly and prematurely when Harry invades Snape's privacy by looking at Snape's memories without permission and gets thrown out of Snape's office and told never to go back. In seeing those memories, Harry finds out about Snape's rivalry with his father, James Potter, while they were both students at Hogwarts. When Snape becomes the only Order member left at Hogwarts after the other Order members all become forced out by the Ministry, Harry gives him a cryptic message before heading into the Death Eaters' trap at the Department of Mysteries. Snape alerts the Order where Harry was going, leading to several Order members going to the Ministry to rescue Harry and his DA friends, thus saving Harry's life yet again.
Snape finally achieves his goal of becoming Defense Against the Dark Arts teacher. Harry ends up with Snape's old potions book, and eventually earns detention from Snape for using the Sectumsempra spell. At the end of Half-Blood Prince, Snape kills Dumbledore and reveals to Harry that he is the Half-Blood Prince whose book Harry has.
This book is the most important for Snape. The book opens with Snape's reporting to a meeting with Voldemort, now based at Malfoy Manor. Snape is Headmaster of Hogwarts during most of Deathly Hallows. He appears to be working with the Death Eaters for most of the book, but when Harry goes into the Shrieking Shack under his invisibility cloak to find Voldemort, he witnesses Voldemort setting Nagini, Voldemort's snake, on Snape because Voldemort believes Snape to be the true master of the Elder Wand (accumulated by his "murder" of Albus Dumbledore), and with this logic, he feels he must murder Snape to become the true master of the Elder Wand at last. As he is dying, Snape calls Harry to his side and he gives Harry all his memories of Harry's mother and with his last ounce of strength forces Harry to look him in the eyes; Lily's eyes being the last thing he sees before he dies. Harry immediately goes and looks into those memories and discovers the truth behind Snape. The memories explain why Snape both hates and loves Harry and explain many of the mysteries of the previous books. Snape's memory is honored when Harry names his son Albus Severus.
Portrayals within films
The adult Severus Snape is portrayed in all the films by Alan Rickman.
He has greasy black hair, a hooked nose and pale skin. His eyes are black and Harry describes them as cold and empty. 
Snape often has a sour personality, especially towards Harry because he hated Harry's father. He seldom smiles, but sneers often and makes snarky comments at every chance he finds.
He shows favoritism towards students in his own House and intimidates the rest of the students. He treats Harry, his two best friends, Ron and Hermione, and Neville particularly harshly, often taking points from them unfairly and handing out detentions far too readily.
He holds a grudge like no other, hating Harry's father beyond the grave, yet in the final book we learn of his undying love for Harry's mother, Lily, and his constant efforts to protect Harry because of his love for Lily.
Magical accomplishments and skills
An expert in Potions and also Defence Against the Dark Arts, as well as the Dark Arts themselves. He is an excellent dueller and was able to invent his own spells for use in combat. He masters the nonverbal spells. He could also improve on the official recipes for Potions that were found in his textbook. He was an accomplished Occlumens, able to keep out even Lord Voldemort. He also learned the trick of flying without the aid of a magical object.
When Snape was a student at school he owned a copy of Advanced Potion-Making by Libatius Borage. In it he wrote This is the property of the Half-Blood Prince by way of his name and wrote throughout the book all his improvements on the Potion instructions and his inventions of new spells.
Snape owns a house on Spinner's End by a river, likely the same house that he grew up in. The river is described as being dirty and smelly, and Bellatrix refers to that neighborhood as a Muggle dunghill. 
Severus Snape was a Half-Blood. His father was a muggle called Tobias Snape and his mother a witch called Eileen Prince. His father is also described as having a hooked nose, and is seen shouting at his mother while she cowers and the boy Severus cries in a corner. 
Depending on how important the character, maybe 0-3 paragraphs here.
In popular culture
Severus Snape is probably best known in popular culture as his character on Potter Puppet Pals, who sings to the sound of the mysterious ticking noise.
Another major portrayal of the character was by Joe Moses in the Youtube sensations A Very Potter Musical and A Very Potter Sequel by Team Starkid, his trademark line being "What the devil is going on here?" and to a certain extent "Detention!"
- "well well well what do we have here"
- "Look...at...me" (DH)
- "Fighting the Dark Arts is like fighting a many headed monster." (HBP)
- "But surely you have noticed that Potter is very like his father?"
- "Yes I have," said Sirius proudly.
- "Well then, you'll know he's so arrogant that criticism simply bounces off him," Snape said sleekly.
- "...Crabbe, loosen your hold a little. If Longbottom suffocates it will mean a lot of tedious paperwork and I am afraid I shall have to mention it on your reference if ever you apply for a job."
- "Fools who wear their hearts proudly on their sleeves, who cannot control their emotions, who wallow in sad memories and allow themselves to be provoked so easily - weak people, in other words - they stand no chance."
- "Master yourself. Discipline your mind."
- ↑ J. K. Rowling, PS chap. 7, (Bloomsbury Publishing paperback, 1997) p. 94
- ↑ J. K. Rowling, CoS chap. 5
- ↑ J. K. Rowling, CoS chap. 11, (Bloomsbury Publishing paperback, 1998) p. 142
- ↑ J. K. Rowling, DH chap. 33, (Scholastic Press hardback, 2007) p. 684; (Bloomsbury Publishing hardback, 2007) p. 549
- ↑ J. K. Rowling, PS chap. 8, (Bloomsbury Publishing papberback, 1997) p. 102.
- ↑ J. K. Rowling, HBP chap. 2, (Scholastic Press hardback, 2005) p. 20.
- ↑ J. K. Rowling, OotP chap. 26, (Bloomsbury Publishing hardback, 2003, p. 521-2; Scholastic Press hardback, 2003, p. 592)
- ↑ J. K. Rowling, OotP chap. 24, (Bloomsbury Publishing hardback, 2005) p. 459
- ↑ J. K. Rowling, OotP chap. 32, (Bloomsbury Publishing hardback, 2005) p. 657
- ↑ J. K. Rowling, OotP
- ↑ J. K. Rowling, OotP