In the 80s it became a widely popular concept to do long cardio sessions at moderate intensity, which was thought to be the "fat burning zone". The idea was that the longer one was in the "fat burning zone", the more fat they would lose.
As the occasional sighting of that god-awful big-haired perm will prove, some people are still stuck in the 80s. While there are many benefits to doing long steady-state cardio (it'll still burn calories, and it'll improve your endurance), science is finding more and more evidence to support the High Intensity Interval Training (HIIT) techniques for maximum fat loss.
Here is the reasoning: Our bodies are extremely efficient at conserving energy, and are always in search of ways to do so. When you do cardio at the same pace for a given amount of time (say, 15-45 minutes), the body starts to adapt and finds the easiest way to keep on doing that activity while expending the least amount of energy. This means that while you are still burning calories, it is no longer the maximum amount you could be burning. With HIIT however, the idea is to switch up the intensity of your cardio (either through speed, incline, or resistance) every couple of minutes. This keeps your body guessing, and by doing that, you force it to work extremely hard in order to keep up with the changes you're throwing at at. Not only that, but your body will continue burning fat for hours after you're finished, due to increased EPOC (excess post-exercise oxygen consumption), which is higher with HIIT than it is with steady-state. One of the benefits of HIIT is that they are usually relatively short sessions. You can burn the same number of calories doing 15 minutes of HIIT as you can in 30 minutes doing steady state cardio. For many people, the time aspect alone is a great enough motivator to switch to intervals!
Now, this is not to say that steady-state is bad! As I mentioned, there are still many benefits to doing this, and if you're an endurance runner/athlete, you have to incorporate it into your training. But if we're talking about fat loss, then the surest way to get the most bang for your buck is to use HIIT.
You can do HIIT anywhere, any time, using any equipment. If you're an outdoor runner, incorporate some sprints into your workouts, or pick a hilly route. If you run on the treadmill, play with the incline or speed (or both) on your machine. If you use the stationary bike or elliptical, change up the resistance. You can also play with the rest:work ratio. In my example, the ratio is 4:1, but as you get fitter you could start doing 2:1, or even 1:1. There are many different types of HIIT training, and I encourage you to look up some sample programs on the internet or in magazines; there are lots to choose from.
Example of HIIT (using running):
5 min - warmup
2 min - run at moderate pace
30 sec - sprint with all you've got!
*repeat 4-6 times
5 min - cooldown
If you've never tried HIIT, then I wouldn't recommend switching your steady-state for it altogether. Try incorporating it into your program once a week, and work your way up to 3-4 days. And don't start with long sessions either - they're meant to be short! 15 minutes (not including warmup or cooldown) is plenty for a beginner! Try it today!