The NFL and NCAA use different size footballs in order to make the game an appropriate level of difficulty for players. NFL footballs are the largest on the market.
College, or NCAA, footballs are the same size as what players find in high school. The NFL plays with a larger ball. The size difference isn't much, but does make a difference in the field of play.
There are several rules that are different between the two levels of sport. The NFL is considered professional and, because of this, the difficulty must be harder to accommodate this level of player. In addition, since college football is considered the unofficial minor leagues for the NFL, it serves as an opportunity for players to build up their skill to accelerate to the top of their sport.