sofi - At my schools gym there several ab benches similar to this
NordicTrack Ab Bench (Equipment - Benches) at Fitness Hownd. In my opinion, however, they are better, but this picture will suffice so you can get an idea of what I do on ab days.
For my first 4 sets, I put the bench on the highest decline and the foot pads as far down as possible. For sets 1 and 3 I use a 45 pound weight and put my feet on the lower pads, so my body is fully extended, my legs are completely straight, and I hold the weight close to my chest. For my first set, I do 20 sit ups (leaving my legs straight still) and on the way back down from each sit up, I come down in a very VERY slow, controlled motion. For set 2 I still use the 45 pound weight, only I move my legs to the higher set of pads. So instead of my body being straight, my waist and legs form about a 60 degree angle and my knees are elevated, forming a 90 degree angle at my knees. I do 10 sit ups here, coming down in a controlled motion again, with the 45 pound weight on my chest. For set 3, I do what I did for set 1, only I do 10 reps. For set 4, I do what I did for set 2, still 10 reps. Between each set, I take a 45-60 second break.
After this, I move the bench so it's completely parallel. I lay straight on the bench and do 10 reverse sit ups. This basically means wih my legs completely straight still, I bend at my waist (using my abs) so that my legs and body form about a 135 degree angle. It's important to try to leave your legs as straight as possible, put your hands under your ass (i.e. don't grab onto the leg pads for support) and bring your legs back down SLOWLY. For my next set, I grab the leg pads and do something similar to the reverse sit ups, only I don't put my legs all the way down, and instead of going straight up and down, I swing my legs in a semi circle from side to side. I do these until I can no longer do them. Next, I repeat the reverse sit ups. Finally, I do the side to side reverse sit ups again, until failure.
All in all, this should take 10-15 minutes maximum, and your abs will be on fire for at least the next 10 minutes, as well as the whole next day (until you get used to the routine). I would recommend for beginners to use a 10 or 25 pound weight (for the first 4 set), as this is a highly intensive ab workout.
Bobby Sands - It's a combination of cardio and ab workout. The cardio will add definition (as you're losing fat), but the ab workout is what will really get the ab muscles bigger and give them the nice round shape. A lot of people say you shouldn't do cardio if you're trying to bulk up, but I disagree with that on the sole basis that it worked for me. In my experience, if you lift heavily and then burn 500-750 calories during your cardio exercise, you can eat a lot (preferably foods high in calories and protein, while being low in fat), which will help the muscles regrow, but you won't be gaining the fat back that you just burned.
Also, I haven't seen anyone mention this, but if you want to get in really good shape, you should do Lat Pulldowns and Chin/Pull Ups. These really build up your back muscles and once you start to lose inches on your waist, you'll have an awesome looking "V" shape (i.e. an attractive shoulder/back to waist ratio).