Amortentia is an extremely powerful love potion which causes the effect of obsession or infatuation. The smell of the potion differs from person to person depending on what they find the most attractive. For example, Hermione Granger could smell freshly cut grass, new parchment, spearmint toothpaste (Film Only) and Ron Weasley's hair (Jo revealed the hair in an interview) however Harry Potter could smell treacle tart, broomstick handle and "something he thought he might have smelled at the burrow (which he realised later was Ginny Weasley's hair.
It is used by mostly girls to attract the ones they like. It is also know to grow powerful the more it ages.
Harry Potter and The Half-Blood Prince
Ronald Weasley accidentally ate Romilda Vane's supposed to be chocolates for Harry Potter.
Ministry Of Magic
J.K Rowling revealed in an interview that in the love room (the room behind the door that melted Harry's magical penknife in the Department of Mysteries) in the Ministry Of Magic, that their was a large fountain of Amortentia.