Virtual hands in FPS MMO game

Hello!

I have a problem and I can't understand how work it.
Hands with animations (shoot weapon for example) aren't the same hands of your real character/avatar. If we are playing a MMO FPS game like Counter Strike f.e., we can see other players with their hands but their real hands are invisible for playing animates and something like this.

Can someone explain me how it works?

Sorry for my english. Thx for read.
Álvaro.