Joysticks are used to operate two or more values simultaneously, that are often (but not always) related to spatial axes.
Modifications
The simplest joystick has a simple handle that can be moved in two dimensions. The sensors get the position of a stick for each of two axes and adjust the output value accordingly, so two values are encoded.
More advanced joysticks in addition to these two axes may contain one or several buttons (and the handle itself may act as a button), sometimes the handle can be also rotated, so more parameters can be encoded.
Size
The joysticks significantly vary by size. The smallest joysticks are operated by a single finger, bigger versions are typically operated by two fingers, and the largest versions are held with the whole hand.