Please explain online differences?

[Munkeh111]

Thanks for responding so quickly, guys.

Unfortunately I really am as ill-informed as I sound. So, beyond the subjective bits I don't understand what you are saying.

Like - there's a marketplace in one and there's a store in another. What are these things?

They are the same thing, the stores are the "Xbox Live Marketplace" and the "PlayStation Store" which contain demos, DLC, small games for download, for playstation there are "semi-games" which are also avaliable on blu-ray and recently full games have been added to the PlayStation Store, such as Burnout Paradise.
There is also a PC PlayStation Store so you can download similar content for the PSP, including full games
Things like wallpapers and themes are also avaliable, but in nearly every case they are free on the Store, and cost on the Marketplace, where you can also get pictures
In certain regions you can get films through these

And - there's a headset, so you can talk/listen to people - but how do you find them? Do you just turn on your whatever-it-is and all these people are just there, or do you have to log on to something, and how do you know who is who and all that sort of stuff.
You just turn on your headset and you talk to the other people in the lobby you are in if you are playing a game. you can also start voice chat with your friends, who you can add to your account
I know it is a lot to ask - but where do I start if I don't understand any of it?