Interline travel is the term used to describe airline employee discounts on air, car, hotel, cruises, and vacation packages. In the airline industry the term also denotes travel between airlines. So, when referring to employee travel, it means that an employee on one airline is traveling on another airline, usually at a heavy discount.
Interline discounts and travel benefits are available for not just the employee...but also for their spouse, children, and parents. Airline retirees are also eligible for discounted travel.
nonrev is short for "non revenue", meaning that airline employees don't generate revenue for their own airline when they fly.