The C button can refer to buttons on the Nintendo 64 controller or a button on the back of the Wii's Nunchuk auxiliary controller.
The C buttons are four small, yellow buttons found on the top right area of a Nintendo 64 controller. All four buttons are used for jumping, and are used to change colors on the character selection screen.
The Nintendo GameCube controller's equivalent of the N64's C buttons is the C-Stick, which is used by default in Super Smash Bros. Melee to activate uncharged smash attacks.
The Nunchuk controller has a C button on its back, which is by default used in Super Smash Bros. Brawl to jump in the Wii remote + Nunchuk control scheme.