A split back formation, also known as a pro set, is an offensive formation that places the quarterback directly behind the center. Two running backs line up near him: one to the quarterback's left and behind him and one to his right and behind him. The running backs, often a fullback and a half back, are placed in close proximity to the quarterback in line with the guards on the offensive line. In regard to receivers, a tight end is placed on one side of the offensive line next to a tackle, and a wide receiver is placed on each side of the formation, farther from the line.
As is the case with many other formations, there are variations of the split back formation. One option takes away a wide receiver and instead uses an additional tight end who is placed at the end of the offensive line.
An advantage of a split back formation is its ability to keep defenses from knowing where the ball is going because of how many options it offers. The quarterback has two different running backs to hand the ball off to as well as a number of wide receivers or tight ends he can target, depending on the personnel in play.
In terms of running plays, specifically, there are multiple options. As an example, the offense can run a blast, or dive, in which the halfback is given the ball and follows the fullback into a hole up the middle, usually for a few yards. Conversely, the play call might be a counter, where the quarterback fakes giving the ball to one running back heading in one direction before handing it off to the other running back going the opposite way.