The concept of outs is to count the number of cards left on the deck (outs) that will improve your current hold cards (together with board) to a hand that will beat other hands.
For example consider you have the hand A♥Q♥
The board comes 7♥3♠9♥
This case is one of the most typical for the outs concept. You need one ♥ to complete your flush, which will be the winning hand (nuts).
Of course you don't have to have a calculator on your side for this. There is a simple formula to commit to memory; The 4-2 rule. For the above flush case you just need to do:
cards that improve my hand on next card (turn) * 2
9 flushes out there * 2 = 18
Why 9 flushes? because there are 13 cards per rank, we hold the 2 and the board has the other 2 (13 - 4 = 9). 18% is a percentage, so you have 18% to hit the flush out there. (Some people likes to add +1 into the above and get 19% but i personally makes it 20% and be done with it).
Now you're knowing the odds you need something to compare to. The whole concept of card odds is to compare them to pot odds. There's no point of just knowing that you have a 18% to win. How to calculate the pot (let's say the pot is 100$ and need to call 20$):
amount to call / (current pot + amount to call)
20$ / (100$ + $20) = 0.16 -> 16%
You card odds = 18% (or say it's %20, no big deal)
Your pot odds = 16%
Since your card odds > pot odds, then you call.
It's not really difficult, it just takes practice.
Another kind of card outs is the straight gutshot, like having K♥J♠ and the board comes T⋄9♣2♠ You're missing the Queen to make the highest straight (nuts) so you're missing 4 cards (outs), so you replace the 9 above with 4 (thus having about the half percentage, 9% to 10% i would like to say in my thought)
but just need to understand looking at purely my cards and the flop how to I know what the winning hand will be and thus the "outs" to achieve it?
Obivously the concept of outs gets interesting (or useful) only with specific type of hands, since you need a considerable number of remaining cards to get something of use. It's a waste of time, to get the outs for everything since everything is not a hand that can win everyone else. You have to get a hand that works well with the board for this.
Also this example you gave:
Say I have 2diamonds and 2clubs and the flop is 4hearts, 9 spades and queen of diamonds.
So 2⋄2♣
on 4♥9♠Q⋄
It's correct that you have 2 outs (the 2 remaining de(o)uches) because these will hit your set. Still there are 2 problems with this: 1) your outs are far too few to even compare 2) there's no guarantee that someone has not a bigger set already. The concept of outs is to try to hit the lock hand (or close to it) In this specific example you just don't calculate the outs; it's a hit or fold. (read about set mining)
