The back row is the rear area, or last three slots, of your party in combat. No character classes do better staying in the back row, but the more fragile ones may benefit from having a few fighters between them and the enemy. Thieves and Mages should stay in the back so they can dole out a little damage without being in range of enemy melee.
Against magic users, front row or back row, everyone will take a beating.
There are a collection of weapons that allow characters such as the Thief to attack from the back row. An example of these kinds of weapons is the Sling.
See also: Edit