Vasalva makes some good points.
This is not the same for everybody, but if you are overweight and young, starting a regular hypertrophy routine that sees you in the gym for an hour, 3-4 days a week is going to assist you to lose fat, while building muscle at the same time. As long as your diet is in order
As always, diet is essential here. If an overweight person can keep their calories at the same level as they always have been, effects will be slow and difficult to see right away. And of course, if the individual is eating a crappy diet, results of any kind will never be optimal.
But with a high protein, low carb, properly calculated diet, a beginner can see quite dramatic results in terms of losing fat and gaining muscle simultaneously.
Losing fat and gaining muscle at the same time is called 'beginner gains' for those with less than a year or so of training and/or who are young. When veterans and oldies try and do it, it's called 're-comping' and it's a slow, steady process where - once again - diet is absolutely key.
Hope this helps.
W.A. AMATEUR STRONGMAN
Axle clean-press: 100 kgs (220 lbs)
Bench press: 135 kgs (298 lbs) - 1st PL meet 16th October 2011
Deadlift w/Barbell: 180 kgs (397 lbs)
Deadlift w/Hexbar: 225 kgs (496 lbs)
Farmers walk: 240 kgs (530 lbs), 50 feet
Front squat: 100 kgs (220 lbs)
Log clean-press: 100 kgs (220 lbs)
Strict OHP: 85 kgs (187 lbs) 3 reps
Tyre flip: 260 kgs (573 lbs), 100 feet