What REDace0 said... except I'm going to explain it in a different way.

There used to be two different types of levels: Physical levels, and skill levels.

Your character itself was one level, and gained experience no matter what job they were on. Your physical level awarded you with points that you could use toward attributes (just like your current levels award you).

Your skill levels were your 'jobs', or 'classes'... You used to gain 'skill points' by using abilities. Each 'skill' level gave you different abilities, spells, weapon skills, etc. Unfortunately, this was bad for mages (who use large nukes instead of small abilities). Eventually, they changed this so that you earned skillpoints by defeating a monster (just like experience points), and it was fair for everyone.

At that same time, 'SURPLUS' sp existed. Basically, instead of being rewarded by being a casual player and taking time off the game (by being given 'rested' exp), you were punished for playing one class a lot. After a certain amount of skill points had been earned, they were reduced by a percentage and reduced even more until they were painfully low. You had to wait a certain amount of days for this to 'reset'.

People complained and were unhappy, so SURPLUS sp was abolished early on. Physical levels were also done away with, as it was determined they were pretty much pointless. "Why fix what isn't broken?" The multiple job system on one character worked in XI... why make it unnecessarily complicated now?

In the future, we will be given 'rested' exp for players that don't play often.