This question may or may not be very stupid, as I am kind of ignorant when it comes to computers. My friend has convinced me to switch to Linux, and I'm trying to figure out how I can game on it.
Any game? No.
From the native Linux client you can only play the games that have Linux releases. In my case this is 86/182 games. Those that are there work well but if you're curious about any one game, go to its Steam page and that should tell you if it has a Linux release or not.
For the others you can install the Windows version of Steam under Wine. This is a compatibility layer which will allow you to play most things. Sometimes with some kicking and screaming, sometimes not at all.
Edit: Linux games in my library has gone from 86/182 to 129/230 in the 6 months since I wrote this. There are many more out there that I don't yet own.
If you download Steam, and install it, and log into your Steam account, you can play any Linux-compatible games on Ubuntu.
There are games that are Linux-compatible, and you will be restrained to playing those.
You also only be able to play games that you buy or are free, but I am assuming you already know that.
There are many games on steam that won't work on Linux, but also there are many that will.
You can browse Linux-compatible steam games here: http://store.steampowered.com/browse/linux/