NOTE : Don't worry, this encyclopedia will soon be extensively updated to include entries from the current fourth volume of "Teenage Mutant Ninjat Turtles," as well as the second volume of "Tales of the TMNT."
Here is a comprehensive (and growing) listing of the characters that make up the rich Mirage/Image TMNT universe, and the details of their comic adventures thus far (and additional information taken from Palladium RPG material). What you see below is from the Mirage/Image universe, just the Mirage/Image universe, and only the Mirage/Image universe. As the comic TMNT has a long and somewhat complicated past, this encyclopedia should help any new readers of the Image series understand what has gone on in the Turtles history books.
For Leonardo, Raphael, Michaelangelo, Donatello, and Splinter, see the profiles section here. For Oroku Saki/The Shredder, see the "Shredder Unmasked" section here. For Klunk, see the "Legacy of Klunk" section here.
NOTE : Shellshock refers to the backstories within the first Mirage volume of TMNT, which were collected in a TPB called Shellshock. TMNT = Teenage Mutant Ninja Turtles and TSD = The Savage Dragon (now known as just Savage Dragon)