When I first started cardio training, all I used to perform was running. I did not do much of it, probably about once a week, and did not do it for long, only ten to twenty minutes at a medium intensity. As time went by however, I started to learn about low intensity cardio, high intensity interval training and I started to use various cardio machines available at the gym. I have found each machine or cardio exercise to be different and below are my personal experiences of them.
Exercise Bike - A tough machine which produces great cardio benefits and strong legs. The only time it uses the upper body muscles is when you push down on the legs as they rotate. Probably not best to use the day before or on the same day of a leg workout with weights, or a strong martial art session which involves a lot of kicks. I have found it best to use a low intensity long duration approach with this machine as high intensity produces too much strain on the legs.
Cross Trainer - A great machine which works nearly all the muscles of the body and high intensity training will not place too much strain on a certain muscle group as opposed to the exercise bike. If you have a joint injury it is a good machine to use to get a cardio workout done without stressing the injury to much.
Rowing Machine - Although people swear that this machine gives them a great workout, for me, it has never done so. I don't feel it produces many good cardiovascular benefits but does provide a good pump in the arms. I tend to stay away from it.
Stepping machine - Similar to the exercise bike in the fact that the legs get a great workout. However, I do not feel that it produces as good cardiovascular benefits as the bike, but when used for two minutes or so with high intensity after a long low intensity session on the bike, it can produce a great burn in the legs which helps condition them.
Walking - I have always felt that in order to feel any cardiovascular benefit with walking, I must perform it for at least an hour and at a slightly faster pace to everyday walking. Personally, I think cardio time is best used up with another exercise.
Swimming - A great exercise that helps tone the muscles while producing good cardio benefits. However, as I do not have a waterproof iPod (something I definitely need when doing cardio) I tend to swim only when on holiday.
Skipping - I find skipping to be great. It really helps tone the shoulders and I have found it to work wonders when performed using a high intensity interval approach. I usually skip at a medium pace for 30 seconds, then skip as fast as possible for 30 seconds then rest for twenty seconds. After about 10 - 20 sets the workout is done.
Running - For me, it is the ultimate cardio exercise. Some days it shall be high intensity interval training, other days it will be low intensity long duration cardio. At home, I mainly use a running machine at the local gym. If I am on holiday I always use running as a way for me to get out, explore my surroundings and enjoy the heat (as long as it's a hot climate). People claim that running is bad for the joints, however, I feel the opposite. When I run, I feel much better for it.
There are plenty of other cardio machines and exercises that people use, but these above are the main ones which I have used and my own personal experiences from them. Others will probably feel differently and I invite you to leave comments outlining your own approaches to cardio training.
Although I favour running, it's always best to constantly rotate the cardio exercise you perform in order to stop boredom developing, to train different muscles from different angles, to train different intensity levels and to become knowledgeable with different machines and exercises. Just as with martial arts, its best to cross train in order to gain as much experience as possible.