When I played in high school you couldn't use your hands at all. However, due to the size and speed of college athletes the rules have modified over time. Most fans watching a game see a hand grab on a jersey and assume that's holding but is usually isn't. Below are the NCAA rules on use of hands:
ARTICLE 3.
a. A teammate of a runner or a passer legally may block with his shoulders, his hands, the outer surface of his arms or any other part of his body under the following provisions.
1. The hand(s) shall be:
(a) In advance of the elbow.
(b) Inside the frame of the opponent’s body (Exception: When the opponent turns his back to the blocker) (A.R. 9-3-3-VI and VII).
(c) At or below the shoulder(s) of the blocker and the opponent (Exception: When the opponent squats, ducks or submarines).
(d) Apart and never in a locked position.
2. The hand(s) shall be open with the palm(s) facing the frame of the opponent or closed or cupped with the palms not facing the opponent (A.R. 9-3-3-I-IV and VI-VIII).