Gamepads generally feature a set of buttons handled with the right thumb and a direction controller handled with the left. The direction controller has traditionally been a four-way digital cross (also named a joypad, or alternatively a D-pad, and never called arrow keys), but most modern controllers additionally (or as a substitute) feature one or more analog sticks.
Some common additions to the standard pad include shoulder buttons (also called “bumpers”) and triggers placed along the edges of the pad (shoulder buttons are usually digital, i.e. merely on/off; while triggers are usually analog); centrally placed start, select, and home buttons, and an internal motor to provide force feedback. Analog triggers, like that of the GameCube controller, are pressure-sensitive and games can read in the amount of pressure applied to one to control the intensity of a certain action, such as how forceful water is to be sprayed in Super Mario Sunshine.
NES B and A face buttons
There are programmable joysticks that can emulate keyboard input. Generally they have been made to circumvent the lack of joystick support in some computer games, e.g. the Belkin Nostromo SpeedPad n52. There are several programs that emulate keyboard and mouse input with a gamepad such as the free and open-source cross-platform software antimicro, Enjoy2, or proprietary commercial solutions such as JoyToKey, Xpadder, and Pinnacle Game Profiler