There are two types of seniority:

Global Seniority: This is by your date of hire at the company, and almost never changes for most of us. The person ahead of you and the person behind you on the list on day one will still be there until one of you resigns or retires. This seniority is used to determine who gets what domicile/base, what airplane, and what seat (CA or FO).

Relative Seniority: Once assigned to a specific base, equipment, and seat you have relative seniority within that group. This is used for bidding trips, days off, and vacation. Necessary because you can't have ALL of the CA's on a senior plane (ie widebody) all taking Christmas off just because they all have high global seniority.

So you can be very senior globally but if you're a CA on desirable airplane at a desirable base you could be working all the weekends and holidays if the other CA's are more senior.

Example: A narrowbody FO with 3 years at the company might have a great schedule and his pick of vacation, while a 30-year widebody CA might have garbage schedules and vacation. But he gets paid three times more, and might be based in a nice town of his choosing. Larger airplanes pay more, and CA's make at least 50% more than FO's.

The nice thing about this system is you can always trade your global seniority to strike YOUR desired balance between location, money, and schedule. Especially at large airlines with numerous fleet types. And you can do this at almost any time, and can mostly go back and forth as desired.

Caveat: This is for US/Canadian airlines only, foriegn airlines don't always do it this way.