It is the seventh novel in the series and follows Harry Potter and the Half-Blood Prince. All the main characters have been introduced in the previous books. If you'd like to start at the beginning, the first book in the series is Harry Potter and the Philosopher's Stone (published in the US as Harry Potter and the Sorcerer's Stone).
As planned in Harry Potter and the Half-Blood Prince, Harry Potter, Hermione Granger and Ron Weasley drop out of Hogwarts to pursue Voldemort.
As expected, this final novel in the series resolves the mysteries. Heros and villains die.