HTML is probably the best-known markup language.
Some tags change the way text looks, others change the way it behaves.
A hyperlink is an example of changing behavior. Most XML languages are used for defining documents. In other words, they allow programs to break up alot of words, pictures and other data into useful chunks that can be processed by a program. UIML, on the other hand is used for defining the actual interface elements. This means the buttons, menus, lists and other controls that allow a program to function in a graphical interface like Windows or Motif.