Here are the answers to some questions you might be asking:

Is Harry Potter and the Half Blood Prince the real title of the 6th book?
Yes, last week JK Rowling revealed the title to us on her website.

Who is the Half Blood Prince?
Now, why would JK Rowling tell us this? It would ruin the whole story. The Half Blood Price is for us to figure out. All we know is that the Half Blood Prince is not Harry, Voldemort, or Mark Evans.

How do you know that the Half Blood Prince is not Voldemort of Harry?
JK Rowling posted the following on her website:

"… giving you further hints about book six. But as a little bonus, and compensation for having been messed around by Mr. or Ms. Storgé, I shall tell you one thing without making you shift any bricks at all: the HBP is neither Harry nor Voldemort. And that's all I'm saying on THAT subject until the book's published."

HBP stands for Half Blood Prince.

How do you know the Half Blood Prince isn't Mark Evans?
JK Rowling posted in the FAQ section of the website that, "Mark Evans is... nobody." This means that he can't be the Half Blood Prince.

What does it mean, Half Blood Prince?
I'm not exactly sure, but I think it means that in the 6th book there will be a character who has half blood and is in someway a prince.

What is a Half Blood?
If you are a half blood, you have a muggle parent, and a magical parent.