In Star Trek, what exactly is the Prime Directive?

Well in The Vengeance Factor,the dispute between the 2 factions(Acamarians and Gatherers)was affecting the Federation as several Federation outposts and colonies had been subjected to Gatherer raids,so in that case(I think)you can make an exception to the Prime Directive as it was in the best intrests of the Federation for the feuding to come to an end!