Lib
Libary files for The Games Factory or Multimedia Fusion -and recently The Games Factory 2 and Multimedia Fusion 2- consisting of sprites. In early ages, libs would be like a playable mini-game in which you could use the sprites if you wish. Over time, libs began to include more and more useless sprites, such as "pregnant Amy Rose". Later on, sprite-sheets were added and rules were made much more strict, limiting libs to only a specific type of sprite per lib and banning the silly mini-game aspect.
Libs have become less and less popular over time due to the wide varity of Lib types and the fact only Clickteam products can use them (Nowadays the majority of fangamers use Game Maker that does not support libs). Whereas clickteam products are fully backwards compatible, they are not forwards compatible, meaning older products such as TGF cannot run MMF, MMFE, TGF2 and MMF2 libs. As clickteam products usually cost, this means many of the libs are not usable without an upgrade, which can cost quite a bit.