Mortgage Brokers act much like insurance brokers or investment brokers/financial advisors - they know the products out there so you don't have to study and scratch your head over the offering of every single bank or lending institution you can think of, going nuts in the process. http://en.wikipedia.org/wiki/Mortgage_broker http://www.wisegeek.com/what-is-a-mortgage-broker.htm They often work on commission, but they have to provide you with something you like, so it's in their interest to not offer up crap you could easily best by hunting on your own.
Many realtors have mortgage lenders they will recommend because they've worked with them before successfully (remember, it's a boon to the realtor to have a well connected, efficient, not-screwing-customers mortgage broker as well, because escrows are less likely to fall through, meaning commissions/fees for the REALTOR happen faster, too).
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http://www.wisegeek.com/what-is-a-mortgage-broker.htm
They often work on commission, but they have to provide you with something you like, so it's in their interest to not offer up crap you could easily best by hunting on your own.
Many realtors have mortgage lenders they will recommend because they've worked with them before successfully (remember, it's a boon to the realtor to have a well connected, efficient, not-screwing-customers mortgage broker as well, because escrows are less likely to fall through, meaning commissions/fees for the REALTOR happen faster, too).