Sprites are the objects that preform actions in a Scratch project. Without them, the only actions that could be preformed would be with the Stage, but because most projects require movement of some type, and the Stage can't move, sprites are used in most projects.
According to the Community Stats on the front page, there's an average of about 6 sprites per project.
Each sprite in a Scratch project has a scripting area, called the Scripts Area. Users can give instructions to a sprite (such as telling the sprite to move) by snapping blocks together in the scripting area. Clicking on the block(s) in the script area will cause the sprite to react based on the function of the block(s) clicked. Clicking on a sprite's thumbnail in the sprite list will bring up the script area of that sprite.
The look of a sprite can also be manipulated by changing its costume. The current costume of a sprite can be changed by clicking on the costume tab and clicking on the desired costume of choice, or by using blocks to select the sprite's costume. New costumes for the sprite can be imported or designed in the Scratch Paint Editor. Additionally, a sprite's costume can also be edited in the Scratch Paint Editor.
Sprites (both code and costumes) can be exported to be imported later into another project if desired. This is achieved by right-clicking on a sprite's thumbnail in the sprite list and then clicking "export" in the pop-up menu.
By default, designing a new Scratch project will provide a scriptless (containing no blocks) sprite in the image of the Scratch Cat.