If I download Steam on Ubuntu, does that mean I can play any game I get on Steam ON Ubuntu?

There is currently a subset of Steam's entire collection available on Linux (viewable on the Featured Linux Games page), however Valve is making a concerted push toward Linux as the OS of choice for gaming, meaning that more will follow.

There are two reasons for this push to Linux,

• They believe Microsoft's Windows 8 is a catastrophe
• They (and a few third party companies) are currently working on a Steam-based console called the Steam Machine, which will come pre-installed with Steam OS, a Linux variant (although it won't be locked down to that OS, you'll be able to install anything on it as you please).

As such, whilst it may not look like there are much games on Linux yet, you can expect more to come.

Steam has a limited number of games available on its Linux platform. You can visit the Steam Linux store to see which games are available.