It seems like this is very roughly the standard across the board:

Employees fly free, or at least really close to it (ZED Low, for example, which is pretty cheap)
Children, Parents, and Spouse (or registered travel companion for unmarried people) get the same benefits, sometimes at a lesser priority (especially for parents) and do not need to fly with the employee
A limited number of "buddy passes" can be issued to friends or other people traveling with you. These aren't free, but have their own rates. As an example, when I worked for Northwest, it cost my girlfriend about $300 to fly with me MSP-SEA-HNL-PDX-MSP.
Airlines also have agreements with other rates for standardized rate plans (ID90, ZED Low/Medium/High, flat service fees) which is reciprocated.

Hope something like that helps! I'm sure others can chime in with their own specifics