1. d4 d5 2. c4
What is a queen's gambit?

It's actually just 1. d4 d5 2. c4; at that point, the most popular moves are to
There are other common move orders as well which can reach the Queen's Gambit position.

As black you can continue by taking the pawn (Queen’s Gambit Accepted), or not take and play something else.
Strangely, the Queen’s Gambit Declined is characterized by a particular series of moves, even though any move other than taking is declining the gambit. The Slav (b3) is technically a queen’s gambit declined, even though it’s not THE Queen’s Gambit Declined. I always found this strange.

Is it d4 d5 c4 dxc4 e4?