There is a short summary of Yenn's story so far. Credits to StaGiors.
"Yennefer of Vengerberg is a Sorceress and the main love interest of Geralt in the books. She was born in year 1173 which makes her about 100 years old by the start of TW3. Her family was a farmer family, that gave her up to be trained as Sorceress, since she was a "hunchback". Her father blamed her half-elf mother for Yennefer's disability. Yennefer is a quadroon, meaning 1/4 elf. During her magical education period, she was described as very talented in magic. A trait that still follows her. After finishing her education, she quickly rose in the ranks of the Mage Community, and was actually the youngest member of the Council of Sorcerers, before the Thanedd Coup, that dismantled the organization.
Yennefer met with Geralt of Rivia, in a rather unpleasant environment, one that involved a Djinn wreaking havoc in a town. Ironically, the witcher and the bard Dandelion, had fished a bottle out of a river, that held the Djinn captive. Dandelion accidentally freed the Djinn, and tried to ask for his 3 wishes. However neither himself or Geralt had realized at the time, that the Djinn was supposed to grant Geralt 3 wishes, since it was the Witcher that held the bottle's seal. Yennefer on the other hand was trying to capture said Djinn, once she learned of it's existence and almost succeeded in doing so. She knew that, in order to capture a Djinn, it needs to not be tied to anyone else. And since everyone though Dandelion was tied to the Djinn, she forced the bard to ask for all his 3 wishes, so that she can capture it. Geralt however realized that it was himself the Djinn was tied to, though a little late at that, which is precisely why the Djinn almost burned the town to cinders. Once he realised it, he rushed to save Yennefer's life, by asking from the Djinn for his Final third Wish, thus releasing it. To both his and Yennefer's surprise, he asked for his Destiny, to be put in the Sorceress' hands. It would appear that the witcher had fallen in love with Yennefer at first sight, however his Last Wish was surprising to both nonetheless. (Thanks go to @wichat for this.)
After these events, the two were involved in a romantic relationship, but a very unstable one. Yennefer at that point, was driven by a desire to find a cure to her infertility. Capturing the Djinn was one of these attempts. Geralt on the other hand, being a witcher, could not stay for long in one place. He had to go back to the Path. So the couple was mostly split. Yennefer was also involved in a romantic relationship with a Sorcerer named Istredd. Geralt and Istredd had a pretty major argument on who should actually be with Yennefer. Nobody quite "won" that argument.
Either way, after some time, Geralt and Yennefer were reunited, and the witcher actually spent a year in her house in Vengerberg. The Sorceress' demanding attitude and his need to be on the road prevailed in the end, and he left without a warning. An action Yennefer had a very hard time accepting. After two years of being apart, Geralt asked for help from Yennefer, since the witcher has problems with Ciri's magical abilities. Problems that the witchers, and Triss Merigold, could not overcome. Geralt had asked Triss for help first, because he would not dare ask Yennefer, due to his previous actions. On a side note, Triss was Yennefer's best friend, and also she had already tried to seduce Geralt with Magic. Which kind of deteriorated the relationship between the two Sorceresses.
Even though Geralt, had referred to Yennefer in his letter, as "Dear Friend", even though he had asked Triss for help before her, Yennefer managed to swallow her pride, and helped Geralt. She abandoned her life in Vengerberg along with all her goals and ambitions and went in Ellander, where she would train Ciri to control her abilities. Yennefer is described as stubborn, ambitious and proud, yet she accepted to help Geralt in his time of need. Ciri is Geralt's Destiny child, and at the witcher's wish, his Destiny is in Yennefer's hands.
In the months she was training Ciri, she managed to create a bond with the girl, something like a mother - daughter relationship. She was a very strict teacher, but also a caring and loving one, in her own way. All this time Geralt was hunting down the people that wanted to catch Ciri.
Yennefer and Geralt met again in Gors Velen, right before the events of the Thanedd Coup. Their meeting started with Geralt explaining himself to her, and Yennefer yelling at Geralt. Or as Dandellion described it, Geralt apologizing, and Yennefer forgiving him. Their relationship was "revitalized", only to die once again, a few days later due to the following events. Geralt was wounded by the Sorcerer Vilgefortz and had a near death experience, Yennefer disappeared, and everyone thought she was a traitor, that helped Vilgefortz, capture Ciri. Even Geralt thought this way. What truly happened, is that Yennefer, acting under Archmistress Tissaia de Vries' instructions, tried to stop the inevitable fight between magicians, with the help of Ciri. Needless to say she failed. She was secretly compressed by Francesca Findabair, and taken away to Dol Blathanna.
After the Thanedd events, the Lodge of Sorceresses was created, and Yennefer was decompressed, since Francesca wanted her to be a part of the Lodge. She was to help the Sorceresses to find Ciri, as she had a strong emotional connection to the girl. Yennefer, having other plans, tried and succeeded to escape. In the following months, she tried locating Vilgefortz, whom she wrongly assumed had captured Ciri. She managed to do so, only to get captured by the Sorcerer. She was imprisoned and tortured, in order to reveal Ciri's location, again by using magic and her emotional connection. She did not cave in. She was much later rescued by Geralt.
Geralt had figured out Yennefer's role in the Thanedd events, and as was to be expected he got back into a relationship with the Sorceress. At that point, Yennefer and Triss convinced Geralt to allow them to take Ciri to the Lodge, so that she can be trained properly. Geralt agreed. Thus the women left to bring Ciri to the Lodge, and Geralt ended up in Rivia. The Lodge, after a meeting, allowed Ciri to see Geralt one last time, before her training starts, so Ciri, Triss and Yennefer traveled to Rivia to see the witcher and inform him of the news. Only to find him stabbed in the stomach by a pitchfork. Yennefer "died" trying to save him. This event in the books is quite vaguely described and it involves time travel and unicorn magic. Either way Yennefer and Geralt ended up in the Isle of Avallac'h as the Witcher 2 storyline suggests. Ciri was nowhere to be found. It is safe to assume that neither Geralt or Yennefer died in Rivia. (Thanks go to @Synvael for this one.)"
Yup Geralt used to be an A-hole alot, and Yenn has sacrificed a lot for him and Ciri. As those triss fans' hatred towards such an amazing independent woman and caring mother, I really could no understand.