Almost always 100% human hair that you can attach to your hair by either clips, glue, or weaving it in to make your hair look naturally long. Sometimes you can buy them synthetic (fake) but it looks more natural real. You will see a lot of famous people with them in for example Paris Hilton. Extensions are in for punk/emo/scene girls for example Kiki kannibal.
"OMG my hair is taking forever to grow, I think I'm just going to buy some hair extensions"
"Did you see Kiki Kannibal's hair extensions? they look bad ass."