The book was published in 1877/78 and is about morality in the noble Russian society of the 19th century. The novel deals with marriage and adultery. Also, it gives a kaleidoscopic overview of the tsarist state in the mid-nineteenth century and addresses the very nature of society at all levels,- of destiny, death, human relationships and the irreconcilable contradictions of existence. Issues of the liberation of the peasants, the policy of the state commissions, legal questions and social life in the aristocratic circles of St. Petersburg and Moscow are discussed. In doing so, Tolstoy proves subliminal humor and self-irony. It ends tragically, and there is much that evokes despair, yet set beside this is an abounding joy in life's many ephemeral pleasures, and a profusion of comic relief.
On a seperate note: A famous legend surrounding the creation of Anna Karenina claims that Tolstoy began writing a cautionary tale about adultery and ended up by falling in love with his magnificent heroine. It is rare to find a reader of the book who doesn't experience the same kind of emotional upheaval: Anna Karenina is filled with major and minor characters who exist in their own right and fully embody their mid-nineteenth-century Russian milieu, but it still belongs entirely to the woman whose name it bears.
Re-read it! I do this every two years, it's a perfect novel about imperfection.