Mark_Robson said:
weight loss = calories in - calories out.
At it's most simplistic, it is as straightforward as that.
What confuses the issue is that we believe we are burning far more than we actually are and then, after exercise, feel hungry and go and over fill. There is an argument for NOT exercising if you want to loose weight - simply because it becomes difficult to manage hunger. I'm not saying become a couch potato, you do need to have a level of general activity, but the high intensity exercise (gyms, cycling, jogging, etc) tends to destabilise the body, throwing it into a kind of binge/starve or wealth/dearth phases. This just confuses the body chemistry and it hoards in times of wealth to deal with the times of poverty.
There are several dieticians that would go further than that, and debate the nature of the calorie consumed (not all calories are equal). We're told, for instance, to go for high carb snacks to replenish glycogen levels... but it is the sugars that spike insulin levels and cause weight gain.
The more I read of this stuff, the more I realise it is a more complex equation than simply calories in and calories out.