(From GNOME guidelines)

A slider allows the user to quickly select a value from a fixed, ordered range, or to increase or decrease the current value. The control looks like the type of slider that you might find on an audio mixing desk or a hi-fi's graphic equalizer.

[edit] Guidelines

• Use a slider when adjusting the value relative to its current value is more important than choosing an absolute value. For example, a volume control: the average user will usually think about turning the volume up or down to make a sound louder or quieter, rather than setting the peak output to a specific decibel value.
• Use a slider when it is useful for the user to control the rate of change of the value in real time. For example, to monitor the effects of a color change in a live preview window as they drag the RGB sliders.
• Label the slider with a text label above it or to its left, using sentence capitalization. Provide an access key in the label that allows the user to give focus directly to the slider.
• Mark significant values along the length of the slider with text or tick marks. For example the left, right and center points on an audio balance control in Figure 6.7, “A simple slider control”.
• For large ranges of integers (more than about 20), and for ranges of floating point numbers, consider providing a text box or spin box that is linked to the slider's value. This allows the user to quickly set or fine-tune the setting more easily than they could with the slider control alone.