Why does minecraft lag so bad when i play multiplayer?

I have a problem and i need help. When i try to join a minecraft server i just fll in darkness for a while and then i will spaw but only half of the world will spawn and then i would leave the server for no reason. I don't think its my internet because its not slow at all when i use facebook or skype. So what is the problem? Is it my internet or is it minecraft? Please help

Added (1). oh yeah i only have 4gb internet and i only use it for minecraft facebook and skype, does minecraft download things or something

Edna

You are missing some info, firstly the specs of your system.
If your single player runs just fine, the problem is your internet connection.
If your single player lags as well, it's your system.

Thunderspeed10000

Maybe it's you're render settings. Go to video setting and turn some animations off change your render distance to tiny and yea. Www.optifine.net reduces lag if you don't have it.

Colin