This week's book giveaway is in the OCPJP forum. We're giving away four copies of OCA/OCP Java SE 7 Programmer I & II Study Guide and have Kathy Sierra & Bert Bates on-line! See this thread for details.
If you are using it in projection to filter out duplicate results, then probably nothing. Some databases actually only support one of them (some support both), so this is more of a syntax issue than anything else.
Otherwise, UNIQUE can also be an attribute for a column making it a candidate/primary key, whereas DISTINCT is only used in SELECT statements.