Generally, if you put down less than 20% then you need to pay PMI in addition to the loan (principle & interest or P&I).
Some more info:
When the loan is paid down to 20% of the original loan amount (not purchase price) the homeowner may request that the PMI be removed with the necessary documentation (an appraisal). The lender will not notify the borrower (as it is not currently required by law) when the homeowner has reached that magic number of 80% loan to value. Legislation is pending currently to make lenders more responsive to borrowers regarding notification and removal of PMI.