In general, use unordered lists for:
Link collections
Short, nonsequenced groups of text
Emphasizing the high points of a presentation
In general, use ordered lists for:
Tables of contents
Instruction sequences
Sets of sequential sections of text
Assigning numbers to short phrases that can be referenced elsewhere
In general, use definition lists for:
Glossaries
Custom bullets (make the item after the <dt> tag an icon-sized bullet image)
Any list of name/value pairs