I asked this question here a few years back.
And it is not a silly question at all.
As bigrockk mentioned, it is like a phone number, but if you aren't registered with ICQ, you don't have to worry.
ICQ is more of a hassle than anything, it's made for the children of this world.
I registered there a couple of years ago and got a virus sent to me.
Take what you like from that.
