Whilst the above is a very good post it could put you off as it makes it sound more complicated than it can be.
Installing most distro's is a very simply process these days. Depending on the distro you can either go through a text based install or a GUI install, some give you a choice (might be separate ISO's) It's not normally a case of guessing what to do, it's pretty much like installing a windows OS.
As far as bash commands go, things move quite quickly in the Linux world and gui alternatives are always appearing, however, the command prompt is a very powerful tool and once you are familiar with it you may find yourself choosing to use that over some gui alternatives.
I would recommend that, as a beginner you choose a distro based on the support available.
I use and would recommend Ubuntu, no doubt other people will recommend other distro's but Ubuntu was my first step into Linux and I'm still using it a year later.
Website:
http://www.ubuntu.com/
Support Forum:
http://www.ubuntuforums.org/
You ask what you can do with Linux, the answer is just about everything you can with mac or windows. The only real limitation is gaming although that is slowly improving. If you're not a big gamer then Linux could quite easily replace your mac and windows os's.
As a final note, I'd like to highlight part of PC Eye's post as it is a very good reason to give linux a try:
Many simply like not getting the adwares, spywares, viruses, malwares. etc. seen with Windows vulnerabilities.
I hope that you decide to give Linux a try and I wish you the best of luck!
Simon