Juugo feels that he needs to be locked away and went to Orochimaru to cure him, but whether Orochimaru didn't or couldn't is unknown. He was kept in a prison lab, bound and locked up. He was befriended by Kimimaro, one of Orochimaru's most trusted servants. Juugo originally saw Kimimaro as a copy-cat, seeing as he was given a version of his curse mark, but Kimimaro told Juugo that he was dying and that it was Juugo's strength that was keeping him alive. Kimimaro seemed to be able to calm him down and he was sad when he found out Kimimaro's body finally gave out trying to ensure Sasuke's retrieval.
Years later when Sasuke freed Juugo, he was initially in his berserker state and Suigetsu fought him off, but Sasuke stopped them. Since Sasuke was important to Kimimaro, Juugo calmed down and decided that he would help Sasuke. But the threat of him losing control scares him and he hopes Sasuke will be able to keep him in check.
Initially mistaken to be a curse, Jugo's ability is actually related to his clan. They can naturally acquire Senjutsu mode and absorbs natural energy constantly. It increases his strength and chakra, it also allows him to transform parts of his body into weapons: such as turning the back of his forearm into a large axe or turning his arm into a type of jackhammer, or formed a jetpack, or a shield and in his full transformation he showed the ability to create energy canons. He also seems to be able to communicate with animals. He could extend his arms to unlimited reaches used to grab falling people.
Jugo is extremely durable. He took a punch from Fourth Raikage (who could crush a cliff with a single punch), and even though the punch had gone right through his chest, and taken his hands, retaliated with ease. He had regenerated afterwards.
Jugo can give chakra to people who are attuned to his powers, like Sasuke, who had previously used cursed seals. He can also use his cells, and his body to heal Sasuke (also need to be attuned to his powers) . Doing this makes him younger, although he is able to grow back to his adult form after absorbing others, although the people he absorbs do not have to be cursed seal recipients or used to his powers.