if im for example using the elites in blood gulch mod and i create a game the people who join it will they see elites to?
Nope. Their map files don't contain the required pc model, biped, sounds, bitmaps, etc. But that's just this case.
Let's say, you mod a pistol bullet damage so that it's a one hit kill(which is pretty cheap, but it's just an example). The game will know the amount of damage to deliver to the person who gets shot. The person will die without experiencing too many problems. This is just meta changing, not a completely different file.
However, there are still problems in meta changing. Let's say you mod the map so you can climb walls. You don't experience difficulties personally because you are the host of the server. But the joiners' map files try to make them play by its own rules, and not by the actual game's. So in this case, the joiner's map file tells the joiner to slip off the wall, but the game is playing by the rules of your map file, and the rules say that you can climb walls, so the player warps back to where the game tells him that he should be(on the wall). So in other words, they go fall, warp, fall, warp, fall, warp(on their screen, that is; on your screen, they're just stuck to the wall). Their game won't crash, though.
Questions are welcome.