I'm 21 years old, 6'3" tall and weight 176 pounds. I eat a good diet, which is high in carbs, low in fat and high in protein.

I've been training for about 5 months now, and I'm ready to take it to the next stage. I'm of a solid build, and certainly not what you'd call fat. I'm looking to be as muscular and defined as possible, so I need to burn off fat. Should I concentrate on building up my muscle, then in a few months start intensive cardio to give me extra definition, or should I train weights and cardio at the same time?

Also, for added defintion, I've been told to stretch my muscles between every set of reps. How much of a benefit would this be?

Doing cardio in conjunction with weight training should be fine if you have the energy, but save the cardio for last on weight training days, so the bulk of your energy goes to muscle building.

Another plan, that works well for me are 6 week cycles. For the first 6 weeks, I eat high carb/high protein, lift 4 days a week and cardio 2-3 days a week. This is enough "bulk time" for me. For the next six weeks, I drop the carbs significantly, lift 3 days a week (going for slightly less weight, higher reps to maintain muscle density) and do cardio 6 days a week (usually in split sessions). That way you can put all your focus into one direction or another. Some people like a longer or shorter time frame, so obviously you're free to adjust it.
Oddly enough, I find that when I am nearing the end of the "bulking" weeks, I push a lot harder since I know it's going to be just maintenence for the next 6.

Naxis