[Ednan]

You use the inbox as a temporary place to put your thoughts.
When you decide where it should be (projects or single action lists) you set that in the action and it goes out of the inbox to the project etc.

The use of inbox is not even needed, it depends on how you want it to work. Anyway, it's just a temporary place, you are not supposed to keep everything in the inbox forever.

EDIT: there's a lot of videos explaining how to work with OF. You can find some here: http://www.omnigroup.com/video/omnifocus and also search in google. Even if it's not for the new OF2 for iPhone, the idea stays the same.