Jumbo loans refer to those that are loan amounts which exceed the maximum amount set forth in Conventional, FHA, and other program guidelines. These amounts depend on the County loan limit set forth by Fannie Mae/Freddie Mac and FHA (Federal Housing Administration). Jumbo programs are not purchased by Fannie Mae/Freddie Mac and are not guaranteed by the FHA, therefore, the credit/income/down payment requirements are much more strict. Most of these loans are sold to the private secondary market, and not the government, and are highly scrutinized.
A down payment of 10-20% is usally the norm, with the vast majority leaning toward the 20% mark. Often times a lender will "portfolio" these types of loans whereby they do not sell the loan on the secondary market, and retain the loan in their own portfolio and service it themselves. The Jumbo market is a necessity as some areas of the country often exceed the maximum loan amount threshold set forth by the conventional and government programs, so, financing does exist for these high end areas.