What causes the lag?

My laptop can play minecraft on the highest settings and not lag at all, but on multiplayer my account lags like crazy to a point where it just falls down a ledge, flies back up and falls repeatedly. I have a Verizon mifi card as my Internet. Is it the server or my connection that causes this?

 

It's usually an indicator that the server is slow, or reaching its limits because of too many people online at once. If your network is stable it shouldn't really be you.