Karin was a guard in charge of one of Orochimaru's prison labs. Sasuke invited her to join his team because of her tracking abilities, but she'd rather ditch the others and have Sasuke all to herself.
Orochimaru found her after her village was destroyed. He asked how she survived and she told him that she knew a large group of people were coming so she fled. Orochimaru then took her with him.
When Sasuke and Suigestu came to recruit her, she was against leaving and said she had to keep the prisoners under control, at which Sasuke told Suigetsu to release them and, despite her protests, released them anyway. But when Suigetsu left, Karin locked the doors and started to get affectionate with Sasuke and tried to convince him that the two of them was all that was needed. Sasuke was slightly startled by her straightforward personality, but was "saved" (note: saved from sex with an attractive female) by Suigetsu whom cut down the locked door. Suigetsu was tired of waiting and wanted to get going, saying that Karin obviously wasn't interested. Karin denied that and said that she's going that way anyway and will accompany them for a short time. She continues to try to find reasons to be alone with Sasuke, and even took his sweaty shirt.
After Sasuke's battle with Deidara,she, Juugo, and Suigetsu found Sasuke. Noticing Sasuke's poor condition, she believed that Sasuke was more powerful than that for being able to defeat Orochimaru, but Sasuke replied that Orochimaru was in a weakened condition. She tended to Sasuke while he recovered from his battle with Deidara and they set out to find Itachi once Sasuke recovered enough. On their way, she used her sensing abilities to both track Itachi and navigate around Naruto, Kakashi, Yamato, Sai, Sakura, Kiba, Hinata, and Shino who were tracking Sasuke.
Once they were near their destination she noticed someone with a massive amount of chakra heading towards them, turned out to be Kisame whom insisted on Sasuke going off to face Itachi alone while he stayed behind to make sure Karin and the others didn't follow.
She and the others met up with Sasuke after his fight with Itachi, where Sasuke re-dubbed them "Team Taka." Here Madara and Kisame conversed with Sasuke about his goals, which Sasuke said was to destroy Konoha. Kisame told Sasuke that he needed more power and Madara said that Akatsuki and Taka should ally, and offered to give Sasuke the Hachibi and Sasuke agreed. Team Taka then ventured to the Land of Lightning to capture the Hachibi's Jinchuuriki, a Cloud ninja named Killer B.
They barely managed to capture him with their lives and they made it back to Akatsuki's hideout. Sasuke decided that he didn't need Madara's help to destroy Konoha so he and Team Taka left, but they were stopped by Madara whom told them that their mission failed and they had to complete at least one mission for him before he'd allow them to leave. So she and the others went to the Land of Iron to kill Danzo, whom was once of Sasuke's targets. After things went bad, she ended up escaping and Madara brought her and Sasuke to confront Danzo himself. She contributed to Sasuke's victory against Danzo.
However in a last ditch effort to win, Danzo decided to use her as a shield, but Sasuke struck her down in order to kill Danzo. Sasuke was about to finish the dying Karin off when Sakura and later Kakashi and Naruto ended up appearing and Sakura healed Karin. Karin remembered when she first met Sasuke, she was a Grass ninja in the Chuunin exams where Sasuke saved her from a bear, but he left when he saw that she didn't have the scroll he needed.
At first she was worried about Sasuke in his condition, but she ultimately decided that she didn't want anything to do with him anymore. After Madara rescued Sasuke, Karin willingly went with Kakashi, Naruto, and Sakura to Konoha. There Ibiki attempted to interrogate her about Akatsuki, she was crying and telling them how she was a victim, but when they weren't buying her sob story she told them she'd cooperate in return for food and something to drink.
Karin has yet to show what she's capable of, but she possesses incredible chakra-detecting abilities. While it's common for ninjas to be able to sense people's chakra, she is incredibly accurate to great detail and can even track people solely on their chakra. Karin also has the ability to heal others just by letting them bite her arm, and suck in her chakra, when Sasuke bit her arm (after being injured by the Eight-tailed Beast) his wounds healed. It should also be noted that she had other bite marks on her arms meaning probably this isn't the first time she used this ability.