Boston is the capital of and largest city in Massachusetts, and is one of the oldest cities in the United States. The largest city in New England, Boston is regarded as the unofficial "Capital of New England" for its economic and cultural impact on the entire New England region.
It is unknown whether or not Boston was affected by the Mist, but it's likely that it was, since David's Scout's radio could not receive any broadcasts from Boston. However, it was also speculated by David (though not proven) that the Mist was simply having a damping effect on radio transmissions.
Sally's parents were staying at her aunt's in Boston when the Mist appeared. Like in the novella, it is unknown whether or not Boston was in the area affected by the disaster.