Airline flight schedules can be based on two different types of assignment: The hub and spoke style, and point to point.
Hub & Spoke
A hub and spoke schedule works for airplanes flying out of one central spot to a destination, and then returning back to the hub. An exception could be a plane flying from one hub to another, in which it will often go to another destination.
Point to Point
Point to point essentially has the airline working like a bus: Instead of having planes originate from a hub, they fly from one destination to another, and continue flying. Southwest Airlines uses the point-to-point system.
Hub & Spoke Pros and Cons
The hub and spoke system is generally cheaper to maintain, and it also helps keep passenger amounts to a maximum level, but it takes longer for people to reach their destinations.
Pros and cons of the Point to Point System
The point-to-point system allows access to multiple destinations, which means fliers are less likely to have to transfer at a hub station. But because the service doesn't revolve around a hub, it's less efficient, especially when a plane flies a route with few passengers.
Legacy carriers such as Delta, Northwest and American Airlines use the hub and spoke system, while some low-cost carriers use the point-to-point system.