In order for a transformation to occur, a Saiyan need to have a tail, and at least 17 million zeno units of Blutz Waves to initiate the transformation. Blutz Waves are found only in light reflected by a moon, and easiest method for it to work is if a Saiyan were to look at a full moon, making the process similar to a werewolf in a way. If no moon is present, or is obstructed by clouds or another planetary body, the transformation is negated. Cutting off a Saiyan's tail also reverses this transformation. During the Saiyan Saga in the manga/anime, Vegeta is able create an artificial moon that "bursts open and mixes" with the planet's atmosphere, when he finds out that the Earth's moon had been destroyed. Turles from the Tree of Might movie also performs this technique to forcibly turn Gohan into an Great Ape. Great Apes are ten times larger and ten times more powerful than their normal Saiyan form. Using this transformation, small groups of 5-6, possibly less Saiyans are able to single-handedly destroy alien planets they intend to conquer.
All Great Apes generally have the same appearance with few exceptions. The only thing that helps individuals tell each Saiyan apart are their clothes (if they have any). They have brown fur, red eyes, pointed ears, sharp teeth, and spiky hair.
Great Apes also have an extremely violent disposition, which usually results in them loosing their reasoning skills and destroying everything in site. Vegeta is also notably one of the only Saiyans to retain their reasoning skills to the point of being able to speak. The only other exception is the Tuffle Baby in Dragon Ball GT, while possessing Vegeta. The Great Ape transformation is also a necessary prerequisite to unlock Super Saiyan 4 in Dragon Ball GT, which happens after a Saiyan is able to turn into thier Super form while under this transformation.
In Dragon Ball Super: Broly, the titular character Broly has access to a Wrathful state while being enraged, which is noted to have the power of a Great Ape without having to transform into one.