Texture
A texture is an image file usually applied to surfaces in maps, or to models. They are usually in either the .jpg or .tga formats. Shaders can be applied to textures for various visual effects.
In Mapping
Textures for use in maps can be found in the various sub-folders of the "textures" folder. JKA's base textures can be found within assets1.pk3.
Jedi Academy and Jedi Outcast only support textures with dimensions (width/height) which are powers of two such as 8, 16, 32, 64, 128, 256, 512, 1024, 2048, 4096, etc.