Hello, I'm new to chess! What exactly is considered an "opening"?

Is it the first 4 moves ? ie:

1.e4 e5

2. Nf3 e4

Or first 3 pair of moves 1. 2. 3.?

Or..?

Besides, why are some called "Defense" and other "Attack"?
When you talk about "Tarrasch Defense" for instance, are you talking about the Black game instead white or what?

Thanks in advance for your help!

-An opening is series of positions starting from a certain point. For example e4 e5 nf3 nc6 bb5 is the start of the ruy lopez complex.

-These openings have no restriction on moves and many variations occur late in the game, some in the teens.
 
-Attack generally refers to a white opening and defense to a black response

An opening is a PREPARED series of positions or moves at the beginning of the game.  The important part isn't how many moves it lasts, but the fact that it was prepared before the game started.

In general I think you can say you play the opening until all pieces are developed (EDIT: meant minor pieces, and rooks connected). After that the middle game starts.

Basically, the opening ends when you're no longer playing memorized moves and instead are planning your positional and/or tactical goals on the spot. For each person, this can happen at different points in the game. Some openings extend all the way to the endgame. One example would be the 'endgame' variation black can play against the Panov-Botvinnik Attack in the Caro-Kann defense.

As a beginner, I think 1.e4 e5 2.Nf3 Nc6 3.Bc4 the Italian game is more likely to be open and is easy to play with.

If you have heard people say "I have to work on my openings" or anything similar they are probably saying they need to work on preparing specific opening lines. At the same time I believe it is correct to say that even if you aren't aware of the theory in a position you are still playing out the opening for at least a few moves.

For example: say a beginner doesn't know the most common responses to 1. e4 e5 2. Nf3 and plays 2. ... Nc6 after taking a look at the board and figuring it out himself. Even though he hasn't done this because he has prepared it, he is still playing out the opening in the current game. Overall I think it is pretty vague when the opening ends and the middle game begins. Generally when more pieces are developed and tactics start developing rather than just strategically developing your pawns and pieces to good squares, that is when I would start to feel the middle game has begun. But it isn't really all that important either.

Your question is probably asked because you've seen videos on different openings or something like that, and in those cases they are opening move orders that can be prepared to make playing easier so you don't have to think of everything on the spot, but instead feel somewhat comfortable the first few moves.

I thinks it's more a combination of memorization as I put it, and also the point at which you stop 'developing' and start coming up with a long term strategy. So in your example, yes, the person is still playing the opening, but no, he's not coming up with the long term plans and goals as he would in the middlegame. He's merely playing the opening by 'winging' it with logical developing moves. And this all goes back to the Caro-Kann example I gave. That opening doesn't end until the endgame phase, so it can't be entirely rested on 'development' to define 'opening'. Rather it has to rely upon both development and memorization.