Typically, triathlons can be categorized by distance as follows:

The triathlon used to be a strictly no-drafting race on the bike course: a certain area to the rear (usually 7-10m) and side (1.5-2m) of each competitor could only be entered when passing, and for a maximum of 15 seconds.
However, this has been changed by the ITU for shorter distances. It is now allowed in many races (such as the olympic race in Sydney}; aerobars have been severely limited in those races due to safety concerns.

On the swim and run course, drafting has always been allowed.