Crit vs. TT

A crit is a high speed race around a closed course for a certain amount of time plus laps at the end. Normally sprinters do well in these races. They are very fast and you are at or above your LT the entire time.

A TT or Time Trial is an individual race against the clock. Basically the bike leg of a triathlon.

Time Trial is a flat-out speed race. Usually started one racer at a time at time intervals (say, every ten seconds) and the strategy is to go as hard as you can over the specified distance. The course is usually a straight point-to-point or an out-and-back with a single turnaround at the midpoint.

A crit (short for criterium) is a multi-lap race run on a fairly short course. Racers start in a pack, and for the most part ride in a pack. Each lap of the course will have several if not many turns in it. To ride a crit, you have to be able to negotiate those tight turns while riding near full speed in a large pack, and in particular have to be able to accelerate quickly out of those turns. A crit will be either for a set number of total laps, or for a specified number of laps after a specified time period (e.g., three laps after 30 minutes of racing).

Fast, easy, and extremely painful if you do them right.  But (IMHO) not as dangerous as crits.