For all controllers, the letters A, B, C, X, Y, Z are absolute references to the respective buttons on the controllers. U, D, L, R are references to directional moves on the joy-pad of a controller. And F or A stand for forward and away respectively, usually used in relation to a fighting game where you need to execute the move based on your position to the enemy. START and SELECT are the respective buttons.
For SNES, 3DO, and Saturn controllers, RT and LT refer to the buttons along the top edge of the controller.
For the PlayStation, R1, R2, L1, L2 refer to the top four buttons on the top face of the controller. Also, T = Triangle button, O = Circle button, X = Cross button, S = Square button.
For the TurboDuo/PC Engine, the roman numerals "I" through "VI" refer to the buttons on that systems controller (most codes will only refer to "I" and "II" since standard controllers for this platform have just 2-buttons).
When you are required to press and/or hold several buttons simultaniously, those buttons will be listed within parenthesis "(" ")" ... ie: "(A+B+C+START)" = press A, B, C, and START at the same time. In some cases you may need to press the buttons down in the same sequence as they appear in the listing.