We have used on-line check in for AirTran and I would guess that they are probably all the same, or mostly.
What you do is get on-line 24 hours up to 90 minutes before your flight is scheduled to leave. There is a special check-in area and you give your code and whatever else it asks (like departing airport). The system will take you along and show you what to do. The inside of the airplane will come up with different colors to show the taken and open seats. You will be prompted to choose a seat for each passenger on your reservation. You will also then print out a boarding pass for each person. If you cannot print for whatever reason the system tells you to proceed anyway and pick up your boarding passes at the airport. I have never had a problem printing so you should not anticipate any. All you have to do at the airport is check your luggage (use curbside if you can) and then go to security. Since you already have your boarding passes there is no need to stop at the airline counter.
It's very easy, and saves a lot of time at the airport.
