Nobody likes being the last person to board an airplane. In addition to zero overhead bin space that's left over, it's inevitable that the only seat left on the aircraft is in the back next to the lav between two enormous, smelly, sleeping wrestlers.
Traditional airlines get around this by assigning seats at ticketing (and often separating select seats out for elites,) but Southwest Airlines has always had an "open seat" policy. That is, you board roughly in zones and pick whichever seat you want when you get onto the plane. Kind of like a bus. Depending on when you check in and a few other factors, you're allowed to stand in a certain part of the line.
to read more, visit: http://www.gadling.com/2009/09/02/southwest-airlines-offers-early-boarding-for-10/