The first time, she travels back 200 years by touching one of the stones, but doesn't know that's what will happen. She spends approximately three years in 18th century Scotland, meets Jamie, and is forced to marry him to keep her from becoming a prisoner in an English prison, but falls deeply in love with him.
She goes back through the stones to her own time as the Battle of Culloden begins in 1746. Jamie makes her go back because he believes he will be killed in the battle. She knows the outcome because she's from the 1940s, but doesn't know his specific outcome. She never studied history that much in detail. She's pregnant with his child, so he makes her go back to her own time to keep her and the baby safe. Plus, he knows she left a husband behind in her own time, and wants her to be happy and have a chance at a life.
She and her husband in her own time raise Jamie's child. In the TV show we get more of their life in the 1940s through 1960s than we do in the books. It's not a happy marriage, but they stay together for Brianna's sake. Once Brianna graduates from high school, Frank and Claire talk about divorcing, but Frank dies. He dies in the books, too. When Jamie and Brianna go back to Scotland for the funeral of someone she and Frank were close to, they begin researching who lived after the Battle of Culloden. She eventually finds out Jamie survived, and is still alive 20 years later in his own time. Her daughter encourages her to go back and find him, so she does.
It's entirely under her control in both the books and the shows so far, that she makes the choice to touch the stone that sends her back to her own time, and then back to the 18th century again. It was only the first time she didn't expect it to happen.