Circuit training can be very effective.  It shouldn't be the only type of workout that you can do, but a couple of times a week can be excellent for fat and calorie burning, strength building, and improving your cardiovascular system.  There are many types of circuits you can do.  Here are 3 types of circuit workouts.

Set Time Density- Perform the exercises you pick as many times as you can in a given amount of time.  Here's an example of this circuit:  Do 30 push-ups, 15 jump lunges, and 5 pull-ups.  Then start over.  You go through this as many times as you can in 10 minutes.  You'll know your fitness level has increased as your density increases.  (This means you are able to do more sets of this in 10 minutes.) You can rest as long as needed, just make sure you go the full time you set originally.

Variable Time Density- The difference between the Variable Time Density workout and the Set Time Density workout is that here you do 4 sets of the example I gave previously and see what time it is at the end.  You know you improve when your time goes down.  Remember on both of these that you can rest as long as needed.  This one you just don't end until you complete the number of sets you set for yourself.

Set Rest Circuit- In this type of circuit you don't rest more than the allotted time you set in the beginning.  Here's an example.  Perform Burpees for 30 seconds, rest 30 seconds, jumping jacks for 30 seconds, rest 30 seconds, repeat from the beginning.  You would also set how many times through you will do this routine before you start.  In this example you can perform the exercises for a set number of seconds or reps like the first two examples. (The first two examples have to be done with reps because the variable is time and if you say you'll do an exercise for 30 seconds in that one then you can't improve.)  A major difference between this one and the previous two types of circuit workouts is that here you rest no more than the time allotted.

Another way to modify the previous three circuits is to use heavy weight instead of body weight.  Perform heavy squats or heavy shoulder press or heavy bench press for a set number of reps (5) or set number of seconds (10).  Also, remember the key on these is speed.  Slow reps will not get you the results you are looking for.  This means you need to choose exercises you are good at keeping good form.  That being said, form is important, but can be sacrificed a little for speed.  It can't be sacrificed a lot, but a little is fine.

These circuit routines would go perfectly along with my book The 6 Week Workout Program.  If you are interested in that please click here.