The slider, or the track bar, allows the user to change or set a value. This is commonly used for changing something in an interface, such as the volume. The change is immediate in the interface when the user has changed the setting in the slider.
It commonly consist of two parts; the track and the thumb. The thumb is the part which can move, and the track shows the extent and how the thumb can be moved. The difference to switches/toggles is that sliders consist of a sliding scale whereas toggles commonly only have two modes, commonly on/off.
Also see switches.