Texture files

The terms texture and sprite often get confused in game development, so it's worth making the distinction: a texture is simply an image file, a big list of color data telling the interpreting program what color each pixel of the image should be, whereas a sprite can be seen as the 2D equivalent of a mesh—it defines how and where the image will appear in the game scene. Usually, a sprite is just a single quad (a pair of triangles combined to make a rectangular mesh) that renders flat against the current camera.

There are also things called sprite sheets, which are large collections of individual images contained within a larger texture file, commonly used to contain the animations of a 2D character. These files can be split apart by tools, such as Unity's Sprite Atlas tool, to form individual textures for the character's animated frames.

Of course...