I am merchant hoping to accept BTC as means of payment, I need someone to throw more light on the term "DOUBLE SPEND" & how it affects my business
The easy explanation for this goes as follows; someone sends a transaction and then "refreshes" their wallet so that it doesn't register the amount as being spent, and they send the same transaction to another address. The best way to counter this is to require 3-4 confirmations on a transaction before accepting the transaction as completed, as this makes sure that they do not have the ability to double spend and get away with it.
It shouldn't affect your business so long as you make sure the confirmations go through properly.