dom tubing?

I am trying to find some info on the differences in dom tubing. I was able to pick up a tubing bender off of a guy today which included a 180 deg 1 5/8" die. I have found several different kinds of tubing, but the numbers don't mean a lot to me. I understand what dom is, but when would you use 1020 vs 1026 vs ....? I have searched and found some things through google, but need some practical advice. I am planning on eventually doing a cage and some other projects. Thank you in advance.