For all it's complexities, the Prime Directive is extremely simple:
Do not interfere with the development of other cultures
General order number one simply says:
Obey the prime directive
However the prime directive becomes a little more subtle in it's application.
- Contact with pre-warp civilisations is strictly forbidden1
- Contact with warp capable cultures is acceptable, but interference in their culture if forbidden
The prime directive is so important that captains and crew are expected to die to prevent dis-obeying it. Several ship crews have been lost, where they could have saved themselves by escaping to a pre-warp world, because they had to prevent contact at all costs.
This also means that a Starfleet officer is bound to obey all local laws when planetside.
However that prime directive also has terrible consequences. Captain Picard was forced to watch millions of tribal hunter-gatherers die when their planet's atmosphere consumed itself, unable to prevent it or save them.
Federation citiznes are expected to follow the prime directive, but it remains a predominantly Starfleet principle. Some other races, like Vulcans, obey the principle. In fact it is a Vulcan principle. Other races, like Klingons do not have anything even remotely similar.
Prime Directive FAQs
There are several questions raised about the prime directive.
If (illegal) contact has already been made, can Starfleet officers further break the directive?
Technically, they are permitted to make contact, if they believe this will minimise the damage caused.
When Captain Janeway contacted the Occampa, wasn't she breaking the directive?
While the Occampa did not possess warp technology, the Caretaker had already made contact with them and was supplying them with energy to stay alive. Janeway may have decided that their culture had already been contaminated by outsiders. In addition, their technology level was already very advanced, possibly going beyong the 'warp level' required for contact, even though they had yet to achieve actual warp flight.
Didn't Kirk disobey the prime directive a lot?
Yes, constantly, but always for a greater good, such as to defeat Klingons.