Here's the one I use as a definition:
"When contacting a planet making normal progress towardsa free and unified society, a Starfleet Officer shall make no identification of self or mission; No interference with the social development of said planet; no references to space, or the fact that there are other worlds or more advanced civilizations."
--The Primie Directive of Non-Interference, Revised 2278
By in large, Federation crews are expected to follow this to the letter. There are however, a few exceptions to the rule when Contact can be made:
1) Interference has already happened: this covers instances when said culture has already suffered cultural contamination by accident from Starfleet or Third party influence. The logic being is that the damage has been done, so you might as well try to help.
2) Natural disaster: this is a relatively contraversial option and the Fleet and Federation is rather split on the Issue. Does extinction by natural events count as 'natural development' or does the Federation have a moral duty to help in the event of natrual disaster.
3) Artificial Disaster: Disaster caused through an Unnatrual agency such as a starship or weapon impacting into said planet. For the rational, see Exception One.
Aside from these three caviats, the Federation and the Fleet are pretty hardline on enforcing the Prime Directive. and ti give starship commanders enough discretion to go down to planet side and initiate contact if needed. And no excuse to sit by and watch planets die because of The Prime Directive.
Except for those who are too towrried about thier careers to save a planet of innocents. And eys that was a shot accross the bow ofa certain bald paited captian.