A conventional loan is a mortgage loan that conforms to Fannie Mae or Freddie Mac guidelines and is also NOT insured or guaranteed by HUD (FHA Loans), VA or USDA. A jumbo loan is very similar, but for loans over the conventional loan limits.
Like all loans, they can be used for the purchase or refinance of a home. The most well-known guideline for one of these loans is the loan size limit, which is set by the Federal Housing Finance Agency (FHFA) and can be found on the Fannie Mae website. Other guidelines include borrower’s equity position, debt-to-income ratio, credit score and history, and property type to name a few.
A loan which does not meet these guidelines specifically because the loan amount exceeds the guideline limits is known as a jumbo loan. Jumbo loans can also have other stricter requirements such as higher credit score requirements, lower income-ratio figures and others depending on the details of the transaction.
The documentation requirements are just about the same for all of these options. List of documents needed can be found here.