Book Seven of the Harry Potter series will be the last in the main series, although the author, J. K. Rowling, has reportedly considered writing other books that play a secondary role. Rowling has said that by the end of this book 'you'll have all the back story you'll need', and that a prequel to the series will therefore be unnecessary. If the publication timing is similar to previous books, the final installment will likely be published sometime between 2007 and 2009.

In an interview following the release of Harry Potter and the Half-Blood Prince, JK Rowling stated that

Realistically, I don't think I'm going to be able to do real work on it until next year [i.e. 2006]. I see next year as the time that I’m really going to write seven. But I've started and I am doing little bits and pieces here and there when I can. But you’ve seen how young Mackenzie still is, and you can bear actual witness to the fact that I do have a very small, real baby, so I'm going to try and give Mackenzie what I gave David, which is pretty much a year of uninterrupted 'me time,' and then I'll start writing seriously again.

Statements by the author

Rowling has revealed that she has already written the last chapter of this book, although she admitted she would probably have to change some of it. It will reveal what happens to the surviving characters after they leave Hogwarts.
The last word in the series is currently 'scar', though Rowling has admitted that it might change as she finishes the novel.
Book 7 is almost like part two of the sixth book, Harry Potter and the Half-Blood Prince, and may thus provide a direct continuation of the plot of that book.
The Potters' profession is important to the plot, and there will be a huge revelation about Lily Potter.
It is very important to the plot that Harry has his mother's eyes, and that her wand was good for charms.
There will be no scene in which Harry or anyone else is playing Quidditch.
Rita Skeeter will appear again, as will Dolores Umbridge.
Dumbledore's brother Aberforth will probably appear (Rowling has confirmed that he's the barman of Hog's Head) and more about Dumbledore's background will be revealed.
We will find out exactly why Rowling had Sirius killed. The magical mirrors that Sirius gave Harry will appear again.
We will find out, what the "look of triumph", which Dumbledore had at the end of GOF, was about.
We will find out exactly what Dudley saw when he encountered the Dementors.
We will learn about the correspondence between Professor Dumbledore and Petunia Dursley before the death of Harry's parents.
More about Harry's scar will be revealed.
The fact that Voldemort now has Harry's blood in him will be significant.
The choice between what's right and what's easy will be a pillar of the plot in Harry's last 2 years at Hogwarts.
According to J.K. Rowling in the Cub Reporter Press conference following the release of The Half-Blood Prince, the Weasleys find themselves "in the firing line" as a direct result of their blood traitor status.

Direct evidence in The Half-Blood Prince

The end of The Half-Blood Prince provides some clear indications that Book Seven may not center on the established Hogwarts routine (Hogwarts Express, Sorting Hat, Quidditch, appointment of replacement teachers) in the same way that the previous books have: Hogwarts is closed indefinitely until its Board of Governors can make a decision regarding its future, and Harry tells Ron and Hermione that even if it does reopen, he does not intend to return, instead focusing on finding and destroying Voldemort's four remaining Horcruxes and ultimately challenging Voldemort himself. JK Rowling had said that each book is one of Harry's years "at Hogwarts." It is likely Harry will spend some time "at Hogwarts" during the book. The amount of time is not known.

A list of events that The Half-Blood Prince has strongly suggested will take place in the 7th book, based on the immediate plans of Harry Potter and his close friends:

Harry stays with the Dursleys for a brief period of time, leaving them no later than his 17th birthday on July 31, when he comes of age in the wizarding world. (Hermione turned 17 in September during The Half-Blood Prince, and Ron turned 17 in March.)
Over the summer Harry, Ron, and Hermione attend the wedding of Bill Weasley and Fleur Delacour.
Sometime in August, Harry and Ron become eligible for a licence to Apparate.
Harry, Ron, and Hermione visit Godric's Hollow, including the graves of Harry's parents.
Harry, Ron, and Hermione seek out, in order to destroy, the remaining Horcruxes of Lord Voldemort.
A final confrontation between Harry and Voldemort ensues.
In between, it seems most likely that Harry will discover the identity of R.A.B. who may have already destroyed one of the remaining Horcruxes, and may have one or more confrontations with Severus Snape, Peter Pettigrew and Bellatrix Lestrange. There is some debate as to whether Snape betrayed Professor Dumbledore or followed his orders in the previous book — it is expected that the seventh book will reveal unambiguously which side Snape is truly on.

Peter Pettigrew is indebted to Harry Potter for saving his life by convincing Lupin and Sirius to send Peter to Azkaban rather than kill him in Harry Potter and the Prisoner of Azkaban. Dumbledore told Harry that when a wizard is indebted to another, it creates a special bond between them; as such, Harry may be glad to have saved Pettigrew, who may help him in the seventh book.

Bellatrix killed her cousin, Harry's godfather Sirius Black, and tortured the parents of Neville Longbottom; as a prominent minor villain in the series, it is likely that her fate will be revealed.

Possible series follow-ups

Although there are no plans for an eighth book in the series, the author has suggested the possibility of writing an 'encyclopedia' of spells, people, etc. and/or more school textbooks such as her previous Quidditch Through the Ages and Fantastic Beasts and Where to Find Them after Book 7.