Minecraft wiki entry for TNT_cannons, associated text: The charge is a large amount of TNT that propels the shot. All of these TNT blocks must be ignited simultaneously.
The wadding block can be any type of block (except obsidian, bedrock and any other blast-resistant blocks) and provides a buffer between the charge and the shot. Sand, snow, wool and other soft blocks seem to produce the best effects.
The shot is a block of TNT or the user if you are on creative mode and drop into position on time.