Let's start with the basics...
Technically you could call any apartment building or condo tower a Multifamily property. But when I refer to "Multifamily" properties on this site I'm specifically talking about residential real estate properties with 2, 3 or 4 permitted living units. A Duplex has two units, a Triplex has three units and a Fourplex (or Quadplex) has four units.
These 2-4 unit multifamily properties are popular with investors because if you're approved you can get the same conventional financing on them that you could get with a single family home. If you plan on occupying the property yourself you may even be able to get FHA financing (talk to your lender for more information).