This way, a screen full of graphics may require only 1 or 2K of RAM memory. Each character will typically map a 8x8 pixel character cell where one byte of the character memory specifies the pattern and another may specify the color. Phoenix is an example of a character based video game. Since the entire screen is mapped to only 1 or 2K of RAM, a slow 8-bit CPU is able to handle screen updates without having to modify very much memory. Since character graphics tend to be rather chunky and are not addressable on a pixel by pixel basis, one solution is to add what are generally called sprites.
Sprites can number from a few to a hundred per game and are usually used for the moving objects.
Sprites also have transparent parts so that their edges don't look like square black blocks and objects 'behind' them can be seen through holes in the object such as eyes. Mappy and Jr. PacMan are examples of games where characters can have priority over sprites. The sprites have priority over each other as well. A write to video memory causes the flag for that character to be set. This is one of the reasons my PacMan emulation is so fast.