DOMS is Delayed Onset Muscle Soreness. It's what happens after a workout, especially when you've been slack like me and not worked out for a few months.
You're fine the next morning, then things gradually start getting tight -- say, 18-24 hours after you worked out. Not only is it soreness and tightness, but it's a whole feeling of restlessness. It's kind of annoying.
It's particularly bad when it happens in your legs, since you walk funny for a few days.