Interline Discounts ... In a nutshell

A short summary about interline basics...
"Interline travel" is a phrase somtimes employed when talking about the discounted rates that employees of an airline get when traveling on airlines, rental cars, hotels, cruise ships, and also resorts and vacation packages. Within the airline industry, "Interline" also means travleing across two or more airlines. As such, "interline travel" when referencing airline employee travel means that an employee on the first airline is flying on another air carrier, often at a heavy discount.
Additional Information on Interlining
Most Interline discounts and travel privileges are leverageable for more than just the employee . Often, the discounts are also for their spouses, minor children, and parents. Many Airline Retirees are typically able to make use of discount travel.

Further, "nonrev" is an acryonym that stands for "non revenue". This means that airline employees, while they travel like this, don't produce profits for the airline when the employee goes on a trip.
A interline or Non-Rev nonrev , in airline speak is slang meaning a non-revenue producing passenger. Typically this means an employee, or relative of the employee, that is using travel benefits to take a flight on a standby basis at no cost, or at a greatly reduced cost. Subload (Short form of Subject-to-load) is another word meaning nonrev. An additional term is interline travel, which usually means that the employee is using another airline (other than his airline) to fly. The nonrev do not need to be interlining travelers.
When using an interline ticket, the passenger is not able to reserve their place. Since the nature of the ticket is "Standby", contrary to revenue passenger with a ticket is called "Positive Space" or "Confirmed".
Managing The Priority List
If more passengers on standby than seats available, the seats are assigned based on priority: and interline passengers have the lowest priority on the flight standby list. Additionally, interliners will be prioritized against each another, with airline employees prioritized by seniority, time of check-in or both. Next are their family members, and then last in line, "buddy passes".
Closing on Interline Discounts
Although travleing is space available and prioritized, this type of travek remains the most sought after benefits for airline staff. As the air carriers have tightened their belts, however, the amount of available space a seats will shrink each year, putting a ding in the benefits of this "free" travel. However if the employee is patient, and does good planning, it's a terrific way to see the whole world without spending their whole paycheck.
Resources With Additional Information
Interline Travel

Interline Cruises
Interline (wikipedia)