And the numbers and letters you see on your boarding pass are no random combination. Every airline uses a specific system to ascribe letters and numbers to every flight.
The letter component of the flight number is fairly straightforward: They represent the carrier. For example, Delta uses DL, American Airlines is AA, and United is UA.
The system gets more complicated with the numbers. Although each airline has their own rules about how to assign numbers, no airline can use more than four digits for the flights. Every single flight number must be from 1 to 9999.
Throughout the industry, as Mental Floss notes, even flight numbers are typically assigned to north and eastbound flights while south and westbound flights end in odd numbers. (There are some exceptions to the rule.)
Airlines typically assign the return flight number as one digit higher than the outbound flight. For example, JetBlue has a flight from JFK to LAX that's JBU523. When it returns to JFK, the flight number is JBU524.
Generally, the lower the flight number, the more important that route is to the airline. Delta operates DL1, a route between New York's JFK and London Heathrow. The very important flight number likely relates to the airline’s history. Delta’s very first international destination was London in 1978. (Although, back then, the flight operated from Atlanta.)
There are a few flight numbers you are unlikely to ever see. Flights won’t ever have a number like 757 to avoid confusion with the model of aircraft. And, due solely to superstition, airlines won’t operate flight 13 or flight 666 — unless it’s an elaborate joke.