PMI stands for Private Mortgage Insurance. You have to pay PMI if you get a mortgage without paying 20% down payment. PMI usually works out to around 10% of your monthly mortgage payment. To add insult to injury, PMI is not tax-deductable, so it's even more painful.

There are ways of getting around having to pay PMI without putting 20% down. Lenders can break up the mortgage into two mortgages, which somehow gets around this rule.

If at all possible, you want to avoid having to pay PMI. If you do get into a position where you have to pay PMI, as soon as you build enough equity, you should refinance to get rid of the PMI payments.