What exactly is pair bonding?

Perhaps someone can give me a plain english detailed explanation of what pair bonding is. Here's what I think I know (correct me if I am wrong)... Most phone wiring I have ever done have 2 twisted pairs (2 tip and ring's). I presume phone companies set it up this way so each customer can have 2 separate phone numbers. A fax line and a regular phone line or just 2 phones with seperate numbers. Now is pair bonding just combining those 2 lines? Or is it that you get what 2 separate customers would get? I know it's kinda the same thing but then again it's not because I think most places are already set up for 2 phone lines. If I it's just combining the 2 phone numbers that each premise can have, why can't pair bonding be done in MDUs? Thanks