Just saw Half-blood prince

 Spoiler

I don't understand this part. I haven't seen the movie yet of course, but isn't the Battle of Hogwarts in the 7th book? Some elaboration would be welcome. Thanks.

 Spoiler

There's two battles that take place in Hogwarts.

One in 6 (after DEs invade with Draco's help to kill Dumbledore) and an end-all war in 7.

For technical terms, which I think is what confused you, the battle in 6 is properly called the Battle of the Astronomy Tower, though it takes place throughout Hogwarts (especially when they're fleeing). The one in 7 is the Battle of Hogwarts.