I use the clip-on ones to streak the hair rather than make it look longer- it works, just cut the extensions to your wig length, and position the wig's hair to cover the clip, spray the hair in place if you really need to. Usually, the hair naturally falls over the clip, but you never know xD
If you're talking about the hair that you can sew into wigs, they're called wefts.
Fan Expo 2014
Bulma Brief (Dragon Ball Z)
Usagi Tsukino (Sailor Moon Crystal)