During the early stages of Minecraft specifically before version 1.6.1 players could only modify the texture of the game. Hence the term texture packs. However after 1.6.1 there were already modifications which also altered particle effects, sounds, sprites, entities, and lighting. These modification packs were called resource packs.
So, essentially, texture packs only modify the textures while resource packs modify other elements of the game such as audio, effects, blocks and more.