In World of Warcraft, you can gain experience through killing computer-operated opponents, completing canal Gold quests, and exploring new areas. You can gain levels up to the current cap of 70.
To help with this, Blizzard has introduced the Rest Bonus. Basically, what this means is that if you log out in or spend time in one of the large racial cities or an inn for your faction, you will receive an XP bonus when you resume play.
This bonus will give you 200% of the XP you would normally earn for killing opponents. (It does not affect quest experience.) It will last up to a full level and a half if you have been logged out in a rest area for at least a week.
This is obviously great for players who can only get on once every couple of days, as they can pretty much keep up with any friends who are playing daily.
Ultimately, this will depend on your play style of course. However, it has been my experience that a hard-core gamer need not worry overly about the rest bonus.