Goku Character Facts

Goku is known for his cheerful, energetic personality and his love of competition (specifically sport fighting and eating). He has remarkable intuition at seeing the good in others in spite of their actions, though his capacity for forgiving his enemies has occasionally been criticized as excessive; Goku, for the most part, only fights to win, and spares his enemies after defeating them only for them to attack him when his back is turned.[19][20] His staggering power is simultaneously reined in and enhanced by his fierce loyalty and morality. Goku can also be very blunt, and not afraid to express his opinion in a discussion, such as when he questioned Krillin's marriage to Android 18. Despite his kind-hearted nature, Goku can feel incredible anger in cases of extreme turmoil, such as the death of a close friend/relative, or when someone murders many innocents needlessly. He has a simple, practical view of the world and is somewhat naive to the world around him. Because of this, many characters incorrectly assume him to be somewhat stupid, although once he has a basic understanding of things, he learns very quickly. His most remarkable ability is his will to succeed, his tenacity and persistence. He is always determined to push himself beyond his limitations, and never give up even when the odds are stacked against him.
Goku is pure of heart, possessing no negative feelings or thoughts. He is also portrayed to be very innocent as a child, because of growing up in Mount Paozu and not having met anyone other than his grandpa until he meets Bulma. During his childhood, a recurring gag early in the series is that he is incapable of telling the difference between a male and a female without physically touching them, which often gets him into trouble.
Like all Saiyans, Goku can transform into a Great Ape (Oozaru) when he sees the full moon while he possesses his tail, though every time it was not at will. Because this tail was removed permanently in the main series by Kami when restoring the moon, this transformation was never taken by Goku again until Dragon Ball GT (in which it is shown very briefly before the full transition into Golden Great Ape), making it his least used Saiyan transformation. 
Goku achieves this powerful transformation in a rage after Frieza kills Krillin. Frieza had destroyed the Saiyans out of his fear of their growing power and the idea of a possible Super Saiyan. As a result of defeating Frieza, Goku avenges both the Saiyan race and the death of his best friend Krillin. 
After having witnessed Vegeta and Trunks emerging from the Hyperbolic Time Chamber (Room of Spirit and Time), Goku concludes that surpassing the normal Super Saiyan form may be more possible than initially suspected, and enters the room to train himself and Gohan. After teaching Gohan to become a Super Saiyan, Goku is able to achieve this form alongside the further extreme of the form known as Ultra Super Saiyan. 
However, although this new powerup would not take as much of a heavy hit on his speed as the successor would, he decides that staying transformed like this demands far too much ki to be spent, and so decides that the regular Super Saiyan form is the best suited for combat, and makes a plan for him and Gohan to master it before they leave the chamber. 
Like the Ascended Super Saiyan form, Goku achieves this form in the Hyperbolic Time Chamber while training with Gohan. Although apparently stronger than Cell in this state, Goku told Gohan he could not defeat him because Cell was much faster, because "all the power in the world would do no good if you can't even hit the guy." Instead of persevering harder to simply increase power output, Goku decides that simply mastering the Super Saiyan form to suppress its downsides and make it a natural state is the best way to fight and gain power. This form is never used after its initial unlocking along with Ascended Super Saiyan, as a result. 
After realizing the weaknesses inherent to the higher grades of Super Saiyan, Goku concluded that mastery over the basic Super Saiyan form would be more advantageous and went on to improve his endurance in that form, thus achieving this state. 
Although it is unknown when exactly Goku achieved this state of Super Saiyan, it is assumed Goku achieved this transformation in the Other World where he could put unlimited energy into his transformation. It was first seen when fighting Majin Vegeta's own Super Saiyan 2 form in their epic battle during the Majin Buu saga. Goku later transforms into this state before showing the Super Saiyan 3 transformation to Majin Buu and Babidi, and again where he begins to fight Kid Buu. 
Goku achieved this level in Other World as he could put unlimited energy into his transformation. Goku is first seen using this transformation against Majin Buu during the Majin Buu Saga. Once in this form, Goku was able to easily overpower Majin Buu in his first form and stood up to Kid Buu for a fair amount of time. However, the transformation is extremely draining in terms of energy, as it cut Goku's time left on Earth from hours to a stated thirty minutes after performing the transformation for a short time. When attempting to gather enough energy to defeat Kid Buu during their final battle, he was unable to sustain it as he had not taken into account the strain overcoming his living body, and soon reverted back to his base state. Altogether, these heavy disadvantages make it impractical to use the transformation out of the afterlife. 
Goku achieved this form in Dragon Ball GT, after Old Kai helped Goku to grow back his tail which allowed him to once again transform into a Great Ape. When he looked at the Earth during the Baby Saga, its sufficient substitute Blutz waves allowed him to take this form, a combination of the Super Saiyan state and the classic Great Ape transformation. Upon regaining control, he later transformed into Super Saiyan 4, his strongest form yet. 
This form is achieved by Goku after achieving the Golden Great Ape state and being calmed down by Pan. The overwhelming power of this transformation is apparently great enough to overcome the Black Star Dragon Balls' power making Goku's transformation into a child by Red Shenron. The spell is only dispersed during the transformation, however. In addition to him being turned back into an adult temporarily, Goku's overall appearance changes yet again. His tail is easily visible, the hair is longer than when in his normal state (not nearly as long as a Super Saiyan 3, though), his eyes become lined (much like Vegeta's did when he was a Majin in the Majin Buu Saga), his eye color is now golden yellow, and red fur covers almost all of his body from head to toe. His clothes change from his childhood garb, leaving him only dressed below the waist (excluding his armbands). As a Super Saiyan 4, Goku is able to easily surpass all but the most powerful enemies in the final parts of Dragon Ball GT. But at the end of the last episode of Dragon Ball GT, when Goku is an adult again, his tail disappears again and it is unknown if he can ever transform to this state from then on.