Android Roms are different versions of the operating system that one can install when rooted. What are the best? Well its really a matter of opinion but the most popular are by far Cyanogenmod and AOKP. I personally like AOKP and I have used cyanogenmod, aokp, paranoid android, pacman, and carbon. After using all of these I kept going back to AOKP. That is my preference though and downloads for them can be found here http://goo.im/home. Here are a few sites that discuss the features of each:
Infographics showing the features of:
*Note that pacman is not discussed in any of these. It is simply a combination of cmod, aokp, and paranoid. It is awesome but I was only using the aokp features it offered so I figured I would just switch back to the stock aokp.