Well the difference is about capability. In the event that an official boss is somehow incapacitated (Being in prison, or being infirm for example) an Acting Boss rises to fulfill the duties of aforementioned boss.

A Street Boss (when speaking of the official position), however, holds the role concurrently with the official boss. Or acting if the circumstances arise. It is usually number 2 in the family hierarchy.

Now in the generic sense that the media uses the term. It usually refers to a "head captain" that carries a certain favour with the Administration above him and weilds an unofficial authority amongst his fellow captains. Much lower than number 2 in the hierarchy, altho in some situations due to their "down to earth" quality can end up with comparable power to members of the administration.

I never said it was unique to the Colombos, they came to mind first. The Genovese and the Gambinos apparently have the position as well officially. I could name a handful of wiseguys that carry this position officially. However, fingering any unofficial "street bosses" would be a shot in the dark.