Multiple ID's in One Yahoo Messenger

I am using Yahoo! Messenger and I am happy using the same.

I have come across a visible information on chat from a person, while chatting he has used morethan 15 id in a single go? HOW this is possible, is that a crack?

Any IT related Exchangeforum member could answer my question, so that if its a crack, we would avoid those chatting personalities, since I dont support cracks and hacks for bad formalities.