In this post, I mentioned that you can restore a Sql Server backup file to a Sql Server 2005 server.
You cannot do it the other way, though. A backup taken on Sql Server 2005 cannot be restored on a Sql Server 2000 server. If you try, this is the likely message you will get:
Msg 3169, Level 16, State 1, Line 1
The backed-up database has on-disk structure version 611. The server supports version 539 and cannot restore or upgrade this database.