I'll tell ya what it is -- it's waking up in the morning, getting out of bed and going "AAAAAAAHHHH!" It's thinking something horrible happened to you in your sleep, then remembering you worked out for the first time in weeks the day before.
So that's DOMS -- delayed onset muscle soreness. But don't worry, it's a good thing. Your muscles got a pounding and the pain is from microscopic tears in the muscle fibers. How do they heal? By getting stronger.
My DOMS came from the Flat Belly Workout DVD I did yesterday -- the Walk Off Belly Fat workout. It was deceptively effective because I had a lot of fun doing it and apparently got a good workout.
I worked through the soreness with a moderate treadmill workout at the gym this morning, just to get things limber.
Next week I'll check out Flat Belly's other workout: -- Express Belly Blast (ooh, sounds dangerous).