One minor correction to Brandon's otherwise great post:
Incremental backups are available with Standard Edition, starting with 10gR2. DBAs working with earlier releases of SE may find the upgrade to 10.2 worthwhile just for that...
Also beginning with 10gR2, RMAN suports changed-block-tracking. With this feature (enabled), incremental backups only need to read the blocks that have changed; this has the potential to dramatically improve performance of incremental backups. As I recall, this feature is only available in Enterprise Edition.
.... incremental backups, which are only available with Enterprise Edition, don't make as much difference as you might expect because they still require a full scan of all blocks ...
-- Cheers, -- Mark Brinsmead Senior DBA, The Pythian Group http://www.pythian.com/blogs