Lag Question for the tech savvy

Why does it lag when someone goes on the internet in the house besides me? Does anyone know the reason for this, because I have the best ethernet card in my gaming laptop and even if the other internet users don't use the bandwith the game lags just because they use the wifi singnal (I guess). I've experienced the same issue at many different places because I move around a lot. A friend of mine told me for him it does not lag when someone else goes on the internet. Is there anything I can do to prevent it or is it normal?