What is KiSS (Kisekae Set System)?

The Kisekae Set System is a format for creating dress-up dolls. According to Wikipedia, girls' comics fans in Japan invented KiSS in 1991 as a way to create dolls based on their favorite characters and share them on BBS's. KiSS dolls have their own image format and scripting language heavily influenced by the time and place in which they were invented. The images used in the dolls are called "cels" (like animation celluloids) and the dolls are packaged into a Japanese archiving format called LZH. Dolls traditionally have a very limited number of colors -- usually only 256 -- and a pixel art look.

A typical KiSS doll: Spark by Kimiki

For more information, Fox Lee gives excellent summary on their deviantART page.