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.
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/