As I've only recently bought shares for the first time, I've...

As I've only recently bought shares for the first time, I've been hearing of some 'big boys' selling out a huge amount to hopefully drop the value of the stock. Then later that day or next day purchase at a lower price again. I gather this is referred to as 'short selling'? Isn't this illegal and is it governed somehow? If it is governed who's to know if someone sold out a substantial amount to lower the price then his/her friend buys up on a different account?

It has been frustrating as some of the shares I've been looking at, this seems to be happening.

thanks,