The OL tag marks up an ordered list of items. Each item in the list must be marked up with an LI, and each one will have a number in front of it when displayed on the browser screen. The appearance of the number can be changed with the TYPE attribute:
1 - Arabic numbers (default) (1, 2, 3, 4, ...)
START tells where the list starts. COMPACT tells that the list contains only short items, so it can be rendered in a more compacted way.
For more info on lists see the Lists Tutorial.