When you are hired at an airline you are issued a seniority number based on your date of hire. For people in the same class, seniority numbers are issued by either age, birthday, or some other factor (like your SSN).
Once you get your seniority number, everything is based on it (at least in the airline world). Things like vacation, upgrades, schedules, bases, and training order are all based on seniority order.
Seniority is company specific, so if you are furloughed or change companies, you lose your seniority and start all over at the bottom of the pack and payscales.
This is IMO, one of the largest problems with the airline world. There is no "credit" given for past experience. When I was furloughed, I had 4.5 years of experience with a major regional airline, but not enough PIC time to move to someplace like JetBlue, SWA, or another large aircraft operator.
So my options were to go back to start all over again at another regional (often flying with captains that were younger and had less experience that I and for much less pay), or bail out of the airlines and go to a corporate flight department that gave some consideration to experience level.