An unincorporated community is a population hub in the county that does not have a charter as a municipality. It is legally a part of the county in which it is located and receives no additional services than those provided by the general county government.
Where I live in South Carolina incorporated municipalities make up a small portion of most counties. In other states, such as Massachusetts, the county is generally completely carved out into incorporated cities and towns so that there are no unincoporated areas.
According to the US Census an unincoporated community can also be designated as a census designated place. I think of them more as traditionally defined, rather than legally defined, local areas.