Switched Windows to Mac, do I have to buy Steam games again to play on Mac?

Once you buy a steam game, you can use it on any computer, provided that the computer can run it. There should be no need to re-buy the mac versions of the same games, though you will need to download the mac versions of the game to your new computer.

However, not all games for Windows work on Mac or Linux. That is why your mac specific games are less than your total number of games.

There are some workarounds involving getting windows to run on your mac (in virtual machines or something like wine). If you do that, you can install a windows version of steam and play your windows games there, but they are not mac native and may not work as well as you would expect.

When you open Steam on the Mac, your game library is by default shown with a filter for games released under the "SteamPlay" label. Buying games with this label always gives you the Mac version or the Mac and Linux versions for free in addition to the Windows version of the software.

You'll see either of these logos on games with SteamPlay:

(Windows and Mac support)

(Windows, Mac and Linux support)

Sometimes a game publisher might release a SteamPlay version of a game separately from the original, Windows only version. In that case, you will need to buy the new version that includes Mac support.

Reference: Steam Support: SteamPlay

As for Windows-only games, in addition to trying out the open source Windows compatibility layer, WINE, you could also install Windows on the Mac with Apple’s Boot Camp. This has worked well for me with running games with complex graphics that might not be as optimized when running under Mac OS X.

Some games only run on Windows. These games, obviously, will not run on your Mac and do not appear by default in your library. Unfortunately, this is a technical limitation which generally cannot be worked around by giving Valve more money. WINE, as mentioned in another answer, may give you some success, but in my experience, it is very case-by-case in how well it works.

Steam does not make you re-buy games. (I want to say "ever," but I'm not 100% sure there isn't some really weird situation where you could end up buying a game twice). Once you own it, it is attached to your account for good.

You do need to download your games again, though. You might be able to copy the steamapps folder from Windows to Mac, but I don't have experience doing that cross-OS. It does work reasonably well from Windows to Windows, though. At the very least, it might let you just "validate integrity of game cache" instead of re-downloading from scratch.

First of all, any current Mac IS a PC, with its PC architecture and an Intel inside, broadly identical to any other PC save for a small chip saying yeah, dude, you can install that dope OS X in this computer

So if you bought a game for PC you can run it in any of the game's supported operating systems, be it Windows, Linux or OS X (which are the only ones who can currently run the Steam client). They should appear as cute little icons in the right sidebar of the game's store page both in the web browser and the Steam client.

Support may be OS version specific, though.