Like sprinting (not in Minecraft, but in real life). You can sprint for a bit, but then you have to slow down a bit, and then you can do it again for a bit.
If an explanation will make more sense, basically what's actually happening is that the server is trying to send data and information to your computer over the internet. When a message is sent, or someone moves, or blocks are placed, or basically anything changes, the server pulls that data from whatever/whoever sent it (like the player who sent the message or placed the block) and then gives it to every other player, including you. Your internet keeps slowing down suddenly like krugle said, to the point where it can't keep up with all the data the server is sending you; it's just not fast enough. Since it suddenly can't keep up with that pace, your computer will occasionally stop (well, not entirely stop, but will very very very slowly) receiving data from the server and for a second it'll seem like no changes are being made (except whatever you do on your own computer) while your internet struggles to receive all that data from the server--and everybody freezes.
Which, yes, can only be solved by fixing your internet connection, or getting rid of any possible virus.
