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Due to the small size of NES sprites, most moving objects are made of multiple ones. Only 8 sprites can be drawn per scanline, and so the PPU contains an "overflow" flag that is set if more than 8 sprites appear on a scanline.
As noted above, some games (mostly early MMC1 titles such as ''Legend of Zelda'' and ''Castlevania'') store their graphics data in the main PRG ROM. These have a CHR RAM chip instead of a ROM and pass the data from the PRG ROM to the CHR RAM, the main purpose of this being to produce animated background tiles. The arrival of the MMC3 mapper in 1988 eliminated the need for this as animated tiles could now be banked from the CHR ROM on the fly. As the PPU has a 14-bit address bus, it can access up to 16k of CHR ROM or RAM at once.
Essentially, the PPU supports two different kinds of drawable objects: movable (sprites) and non-movable (background). Both kinds of objects are composed of tiles, and moreover a sprite and background object can use the same tile. The difference is that a tile used as a sprite can move around, whereas a tile used as a background cannot. There are no collision detection registers for sprites as was common on most game systems of the era.
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