6x6 is the size of the film as stated above.
C-41 is a film processing type.
Transparencies are developed using several types of development types, including E-3 (older) E-6, K-6 and K-12 for Kodachrome, and most but not all black and white use the standard development chemicals like D-76, Microdol X, lithochrome (older) etc. Depending on the film type, any one of them will work well, but each have their own advantages.
You will need to research the film types and development types to see which ones you may want to use.