Wikipedia contains spoilers
The story is that of a governess, Jane Eyre, who, despite her plainness, captures the heart of her enigmatic employer, Edward Rochester, only to discover that he is already married to a woman who, although alive, is entirely insane.
The early sequences, in which the orphaned Jane is sent to Lowood, a harsh boarding school and witnesses the death of a close friend, Helen Burns, are based on the author's own experiences - two of her sisters died in childhood as a result of the conditions at their school, the Clergy Daughters School at Cowan Bridge. They contain some of the most devastating prose in the English language.
As an aside, Rochester's wife, Bertha, the daughter of a Jamaican planter, is emphatically characterised as being in an advanced stage of syphilitic infection: e.g. "her vices sprang up fast and rank", "her excesses had prematurely developed the germs of insanity", etc. This would necessarily entail that Rochester was also syphilitic, a train of logical consequences which Miss Bronte fails to follow to the conclusion. Despite this, and other incoherences within the novel, it is nevertheless an outstanding and compelling novel which is deserving of its position proximate to the pinnacle of English literature.
The chilling scenes featuring Rochester's first wife have inspired many mystery writers, one of the most obvious spin-offs being Daphne Du Maurier's Rebecca, and a thematic "prequel", Wide Sargasso Sea (1966), which was the most successful novel by Jean Rhys. Another, more recent, use of 'Jane Eyre' has been in The Eyre Affair by Jasper Fforde.