Conventional loans are all loans that are not guaranteed or insured by the Government. In other words, all non-VA, FHA or USDA loans.
They come in two varieties:
All loans that meet Fannie Mae or Freddie Mac’s criteria are called conforming. Those that do not, are called non-conforming.
Most of the conventional mortgages made in the US are conforming; most of the loan programs are non-conforming.
Both the conforming and non-conforming varieties can be for a 1-4 unit property; either can be for a primary residence, for a second home or for an investment property.