http://basketball.about.com/od/collegebasketball101/a/nbavsncaa.htm

Quarters vs. Halves

The NBA plays four 12-minute quarters. NCAA games are comprised of two 20-minute halves. In both the NBA and NCAA, an overtime period is five minutes.

The Clock

The NBA shot clock is 24 seconds. The NCAA shot clock is 35. This is one of several reasons you'll see such wide disparities in scoring in NCAA games - some teams really try to work the clock, play strong defense and end up with final scores in the 50-60 range. Others play up-tempo, hoist a lot of three-pointers, and post NBA-like scores in the 80s, 90s and 100s.
NCAA teams also have a little more time to advance the ball across half-court after a made basket: 10 seconds, as opposed to 8 in the NBA.

Distances

The height of the basket and the distance between backboard and foul line is universal. The overall dimensions of the court - 94 feet long by 50 feet wide - are the same in NBA and NCAA ball as well. But that's where the similarities end.
The most obvious difference - one that you'll notice whenever a NCAA game is played in an NBA arena - is the shorter three-point shot at the collegiate level. An NBA "three" is taken from 23'9" (or 22" in the corners). The NCAA three-point line is a constant 19'9".A subtler difference is the width of the lane, or the "paint." The NBA lane is 16 feet wide. In college it's 12 feet.

Fouls

NBA players get six personal fouls before fouling out. NCAA players get five.
Then there's the tricky part: team fouls. First off, let's differentiate between shooting and non-shooting fouls. A player fouled in the act of shooting gets free throws, but other transgressions - "reaching in," for example - are "non-shooting" unless the offending team is "in the penalty." In other words, a team can commit a certain number of non-shooting fouls per period before giving up free throws to the other team.