1.) Southwest uses colored, numbered boarding passes that you pick up when you check in at the gate or counter. The color corresponds to the flight you're taking. The numbers go from 1 to however many seats there are on the airplane and are given out on a first come, first serve basis. If you get to the counter early, you can get a lower number. WN boards in groups of 30, first boarding group 1-30, then 31-60, then 61-90, then 91+. Many Southwest gates have signs for seperate queue areas where passengers in each boarding group are to line up, in order to organize the boarding process.
If you'll search the archive for "Southwest Boarding Policy" within the last year, you will find several discussions on this topic.
2.) Some of them, but not a whole lot. Since Southwest does not publish aircraft information for individual flights, it's impossible to tell you exactly which flights they're used on. However, from my personal experience, there seemed to be several of the 200s operating flights out of CLE and BWI.