Multi family means there is more than one residence in a single building, like a row home that has been divided into two apartments.
So a duplex or triplex where the whole building is for sale would be listed as a multi-unit. If only one of the residences in that building is for sale it would likely be listed as a condo.
No common areas are shared, there would be completely separate living areas. Multi units are appealing for many buyers, especially if you plan to occupy one of the units in the building. The rents you get from the other units would help pay your mortgage and as long as there are 4 units or less, you can finance it with a residential mortgage. Over 4 units is considered commercial and would require commercial financing.
If I can be of any further assistance, please let me know.
Barbara L. Smith