Progresses through the storylines of Resident Evil 0, Resident Evil and Resident Evil 3 (although the storyline featured in this game is very different to 3's actual storyline. I'm assuming it was too difficult to cut it down, so they mixed it up). There are also hidden scenarios and a few 'chapter' set in 2003, where Chris and Jilltake down Umbrella for good.
Sadly, it does not explore the events of Resident Evil 2 and Resident Evil: Code Veronica. As for the Outbreak games, they aren't in it, but since they aren't 'main' titles in the series it's hardly surprising.
It's worth getting. It's a fun game, and most of the previous 'missing links' in the storyline are filled in.