In America the first book in this series (still going strong after twenty years of ~700 page novels) is known as Outlander, in the UK/Australia/New Zealand the first book is called Cross Stitch. After that as far as I know the sequels all have the same name. The author has spoken at length of how many different ways these books can be categorised (romance, history science fiction etc) but I would say best to describe them as historical fiction with a pinch of salt.
Claire is the main character and she is an English nurse who has just served in WWII when the novel begins with her having a second honeymoon in the highlands. Her husband Frank also served (intelligence). I am not a sci fi reader, not at all, so bear with me here for the next bit. Claire stumbles throw a cleft in the standing stones and finds herself transported about 200 years back in time. As any novel of this length requires, there is a romantic 'complication' (in so far as Claire is already married, 200 years in the future) in the form of Jamie Fraser. Claire attempts to return to Frank and even as she does she is falling in love with Jamie.
The story follows Claire's tumultuous life in the Scottish highlands of the 1700s. It's hard to describe the books without destroying the plot, but I'd love to discuss these books if anyone has read them, or provide more information to people who are considering them. I feel that the 'time travel' factor is sort of misleading of what these books are about. When the author began, she has stated that she was just writing for herself for practice, so it did not matter at the time that she has a woman in the 1700s speaking as a 20th century woman. Obviously later when she decided to publish, she needed to explain the character more. And so the time travel factor was a necessity of sorts. Yes, it is used a bit in contrasts between Claire's two lives, but it not a major aspect of the novels, and they are not travelling through time on a regular basis