The slider class generates a GUI element that allows a user to adjust a numeric value. The programmer doesn't have very much influence on the slider's outfit, there are only very few tags available. Future versions of MUI will probably include some preferences options to allow the user (*not* the programmer) to configure this outfit.
Note that since slider is a subclass of group class, you can get horizontal or vertical sliders by simply using the Slider.Horiz attribute. Default is a horizontal slider.
Here is an XML example of a slider that allows the user to configure a number ranging from 0 to 100:
<slider min="0" max="100"/>