Re: Cardio and Weights together?
First of all, what order do you exercise? I would strongly recommend doing weights before any cardio, otherwise you just fatigue yourself for the weight work.
Getting lean/definition is about carefully balancing your diet/cardio/weights. What you need is a high protein diet so you can maintain as much lean muscle mass as possible, whilst restricting your total calories below maintenance level in order to reduce your bodyfat percentage - this percentage is what you need to focus on if you want visible abs and more definition between muscles.
To get visible abs your bodyfat % needs to be below 15%, ideally nearer 10% which is quite low - most slim people still have a bodyfat percentage between 15% - 25%.
I wouldn't recommend dropping the cardio unless you feel it is definitely hampering your weight training, in which case you could simply do cardio on days you don't lift. However, doing less cardio means you will burn less calories and consequently you will need to eat less in order to lower your bodyfat %.
If your weight is fairly constant at the moment, you can work out your maintenance calories by counting the calories you eat each day for 3 or 4 days, then average this out. To lower your bodyfat you need to gradually reduce your calorie intake. You could do this by dropping 200 calories from your daily intake each week. E.g.:
Week 1: 2400 calories
Week 2: 2200 calories
Until you reach a level where you are losing weight. Don't drop too low too quickly though, or your body will think you're starving yourself and start storing fat which is exactly what you don't want. Do it gradually.
Also be sure to mix up the weight training so you don't do the same exercises all the time. Vary the rep range too so that you get best muscle growth. E.g:
Week 1: Heavy weight (but not max), 8 - 12 reps per set. 4 Sets per exercise.
Week 2: Lighter weight, 14 - 20 reps per set, 3 or 4 sets per exercise.
Week 3: Max weight, 2 - 6 reps per set. 4 Sets.
This way you will be targeting all muscle fibers and should obtain maximum growth.