So I had a conversation with a friend of mine who had recently purchased a modest house. We are both engineers and she could have got more home for the amount she makes (she makes more than me I think

). Well, I was commenting on such a smart choice she made by getting something that she can then double up on payments and lower her principal. Well, she said it didn't work that way (in a snappy tone might I add). We then got deep into this conversation and she began to say how when she pays extra on her payment, they put this money into an escrow account (not really too familiar with this term) and do not deduct it from her principal. WHAT! Yeah, she said more but I was gone after that, I really don't remember it.
What I took from the conversation is that this money goes into excrow and does not lower your principal thus doing nothing to minimize the balance that interest is calculated off of. Is this true? Is this only on certain types of mortgages? Is this something that you can fight? Is it only done on a month by month basis or is all the money you ever over paid get dumped into this escrow account? What is the purpose (if true)? Please let me know because I was thinking about purchasing a home and doubling up on it but I may have to think about this escrow issue.