"Cardio" is the not-so-fond short form name for cardiovascular exercise. Cardio is essential to any effective workout regimen; it's cardio that burns calories, cuts fat, and improves stamina. Without cardio, you can lift a million weights, but you won't lose very much weight yourself!
Cardio is any sustained exercise which keeps your heart rate in 70% or more of your maximum. This includes activities such as swimming, cyling, jogging, and aerobics classes.
The most accurate way is to use a heart rate monitor; most cardio equipment in gyms, such as the treadmill and elliptical walker, have inbuilt monitors which read your heart rate through your palms. You can buy a heart rate monitor at most health equipment stores, such as OSIM and Motion Sports. A less accurate, but relatively well-accepted way to figure out your MHR (maximum heart rate) is to take 220 minus your age, and 70% of that will be your target heart rate zone. E.g if you are 40, your MHR is about 180; 70% of that is 126bpm, your target zone. If you can't sustain that heart rate for a long time, try to stay in the zone for at least 12 minutes. Go easy for a while after that, then try for another 12 minutes. You can exercise at a heart rate higher than your zone, between 80-90%, but though this burns calories faster, it isn't safe to stay in such a high zone for too long, unless you are a very well-conditioned athlete.
The 'talk test' is the easiest way to know if you're in the zone. If you are breathing hard, but not so hard that you can't hold a simple conversation or hum a tune, you're probably spot on!
Ever heard of the term 'skinny fat'? That refers to someone who has a low BMI, but high body fat - skinny, but flabby! This is what happens if you stick to cardio alone and avoid resistance training. Resistance training, including weight-training, yoga and pilates, is what builds muscle. It gives your body shape and tone, so that nothing 'jiggles'; it increases your metabolic rate to help you burn fat faster even when you're just sitting around; it builds your strength and endurance.
Not if you train correctly, you won't. In any case, it's no easy task to become as big as Arnold Schwartzenegger - trust me, I've tried! Women in particular are simply not endowed with the hormones needed to gain huge muscles. If you're wondering if you'll be heavier instead of lighter, muscle is indeed heavier than fat, so you will be slightly heavier than someone else of the same size who doesn't train at all, and unless you happen to be 1.2m tall I can never train you into a 40kg waif. Keep in mind it's not the weighing scale that ultimately makes a difference in the way you look - it's the size of your pants, and whether your butt is jiggling in them!
Whichever diet works for you, and keeps you happy and healthy, is the best diet! Take note of the word 'happy' - if you are feeling deprived, you're not happy - if you're not happy, you are not likely to stick to your diet long, and are more likely to gain even more weight after you've gone off it. Don't deprive yourself of chocolates, char kway teow, or laksa - just don't binge on them all week long either!