Basically, you start off working out of a crappy freight container that is your 'office'. You have to manage things and decide what plane you are going to 'buy' with your starting budget, and what other things you are going to spend cash on, what sort of routes your are going to run etc.
You earn money for delivering stuff and lose it if you screw up (arrive late or whatever), then it's kind of up to you how you expand the business, buy a bigger plane, or a fleet of them, set up more offices, that kind of thing. You have to do maintenance scheduling on your aeroplanes and stuff like that. But the nice thing is that you can basically use any plane you have installed in FS, so you could use a B-17 as a cargo plane it you really wanted to (not that it would be very practical, but there's nothing really stopping you).
The trick is to do 'circuits' so that you can deliver to several airports on a 'lap', that way you maximise your trips. If you like things like EVE and stuff like that, it's not dissimilar to mining and selling stuff or something like that in an MMORPG, but with flying planes involved too.
The next version is set to improve that aspect a bit, amongst other things, since at the moment, it isn't that necessary to do positioning flights. The fact you don't have to do them in the current CP means you can move things along a bit quicker, but people have asked for that feature, so they are evidently putting it in the next version. There is a thread on the Just Flight forums where you can request things you'd like to see in the next version. But just reading a few of the threads on that forum will probably give you a good idea as to whether you'd enjoy it.
It's quite a fun thing to play in my opinion, in that it combines a simulator with an empire building game.
It's really as complicated as you want to make it, which I suppose is the beauty of it. But effectively, it turns FS into a game as well as a flight simulator.
Incidentally, the demo is not FSX compatible, it only works on FS9 (2004), although the full game will work with either FS9 or FSX.
Chock