The Directive states that members of the Starfleet must not interfere in the internal affairs of another species, or the natural development of pre-warp civilizations, either by direct intervention, or the revelation of technology (including the means to create such technology) to the inhabitants. When studying a planet's civilization, particularly during a planetary survey, the Prime Directive makes it clear that there are to be: "No identification of self or mission. No interference with the social development of said planet. No references to space, other worlds, or advanced civilizations..." (TOS: "Bread and Circuses")
Starfleet officers are required to understand that allowing cultures to develop on their own is an important right and therefore must make any sacrifice to protect cultures from contamination, even at the cost of their own lives.
In all, there are 47 sub-orders in the Prime Directive. (VOY: "Infinite Regress")
Originally the Directive was a shield for primitive worlds. If such a world was in danger, Starfleet had been known to order ships to save that world, provided it could be done without violating the Directive (TOS: "The Paradise Syndrome").
The Directive was later amended, prohibiting Starfleet officers from intervening even if it would result in the extinction of an entire species or the end of all life on a planet or star system. By the 24th century the Federation had begun applying the Prime Directive to warp-capable species, refusing to interfere in internal matters such as the Klingon Civil War. (TNG: "Pen Pals", "Homeward", "Redemption", "Redemption II")