I need a way to so that users can register and submit their articles and all those articles go into moderation in the backend. Then it only gets published, once the admin approves. Also, will the users be able to select which category to place their article ?
It is possible yes. When members sign up, the default option is set for them to go in to the subscriber category/role but, you can change it to contributor. By which, they can post articles but not publish them and instead have to be approved. It's a default wordpress option - General settings I believe.
Mixedbag, said it all. Just go to your admin panel and change the settings to anyone can register and set the new users role to contributor.