A conventional loan is a mortgage not insured or guaranteed by a government agency (unlike FHA or VA loans). It's originated and held (or sold) by private lenders, and typically requires a stronger credit profile in exchange for more flexible terms.
Down payments can be as low as 3% for well-qualified first-time buyers, though 20% avoids private mortgage insurance (PMI). Conventional 30-year loans usually carry a somewhat lower rate than FHA or investment loans, and are available for primary residences, second homes, and investment properties (with a larger down payment typically required for investment properties).