Super Saiyan 8 is a Super Saiyan transformation. Goku was the first to reach this form.
Super Saiyan 8 has waist-length, spikey, red hair. Their fur turns black. Their tail turns black. They gain a red aura that is surrounded with red electricity. The first time they transform, or if they are a Majin, they might have an "S"-shaped mark on their forehead.
Although the Saiyan transformation looks evil, the Saiyan has a calm personality. They think about their friends and family more than themselves, and they also sometimes show a bout of confidence, but this is just to intimidate their opponent. If the person reached this through rage, they might be agitated at first, but they calm down seconds after their transformation. If they reached this through more training, they are completely calm, with no fear whatsoever.
Goku was, not surprisingly, the first to reach this form. He reached this form when Meta-Broly started beating his son to death. Watching this, he snapped, and, in a similar fashion to how Goku ascended into a SSj3, reached the Super Saiyan 8 transformation. With his new transformation, he was able to defeat Meta-Broly, although Broly was able to match his speed and strength by the end of the battle, and ultimately forced Goku to win with a Super Kamehameha wave.
Vegeta was the next to reach this transformation. We first see this in the movie Janemba's Revenge, when Goku in his SSj8 form was facing Janemba. SSj8 Vegeta arrived to help out. It was implied that he reached this form through extreme training. We also see Gogeta in his SSj8 form.
Although, by the end of Dragon Ball Z: Friendship is Magic, every Saiyan was capable of reaching this transformation, only Goku, Vegeta, Gohan, Goten, Trunks, and Gotan were truly able to master this form.
This form is capable of giving the user incredible speed and strength. After all, it multiplies the Super Saiyan 7 transformation by 25, or base times 625,000. It also multiplies the speed of a Saiyan by around 10.