The Queen's gambit is very broad... on white's 2nd move you can already call it a queen's gambit and there are many more specific openings it can develop into from there.
One of them is the nimzo indian (but this is not the only one!). So after black plays e6 maybe it will just be a queen's gambit but instead of d5 black follows up with Bb4. If white has a knight on c3 this is a Nimzo. If white played instead say Nf3 then Bb4 by black is a Bogo Indian.
Then of course there are multiple sub variations after that. For example the Nimzo has a Rubenstein and a Classical (and probably 2 or 3 others).
Anyway all that basically amounts to chess trivia. It doesn't help me (or you) play better. Do tactical puzzles and when you study learn endgames, pawn structures, and strategy. All you need to know about the opening is to claim some center, develop, and castle.
You can read posts #2 and #3 at this topic http://www.chess.com/forum/view/chess-openings/should-beginners-play-the-english?lc=1#last_comment as well as check out Estragon's link in post #3 for good opening info.
This topic comes up quite a lot on the forum actually. The advice of very strong players never changes... the opening is the least important phase of the game for new players (and sub-masters in general). It's not why you win or lose games, and the study of it does not build a foundation for further progress in chess.
Like many, I'm a busy man. I have a full time job, two kids, and a girlfriend that always wants attention. Given that, I love chess and want to get better, but I also want to learn openings, though not every one in the book... just way too many variations to memorize.
With all that in mind, I'm looking at (for instance), the Nimzo-Indian from black and in the examples I see generally Queens Gambit Declined. So, again and again I see the Nimzo as a response to the Queen's Gambit. So, my question is this: Given that I recognize I'm basically a beginner, how do I create a repertoire (Ie, work at building an expertise on some minor level), against some of the more popular (most-used), openings? And at what point is the opening classified as a Queens Gambit and at what point the Nimzo-Indian (given the example)? Certainly for the first few moves it may be both, but at point does it merge and become one? Certainly, white starts Queens Gambit and wants to keep it to one of the lines he's familiar with and black wants to keep to the lines of the Nimzo-Indian he's comfortable with. At which point is the game decided which opening is used? I'm a bit confused at that.