A conventional loan is any mortgage that is not insured or guaranteed by a government agency (FHA, VA, USDA). The majority of conventional loans conform to Fannie Mae and Freddie Mac standards, which is why they're also called conforming loans.
Conventional loans are the most popular mortgage product in the U.S. They offer flexible terms (10-, 15-, 20-, 25-, 30-year), competitive rates for borrowers with good credit, and the ability to drop PMI once you reach 20% equity.
Down payment requirements start at 3% for first-time buyers (via HomeReady or Home Possible programs) and 5% for most other buyers. Credit score minimums are typically 620, but 740+ gets the best pricing.