I have a background-image set up and now I want to align my grid to it?
How do I do it. I remember watching a video where I could simply rotate it like an object?
But if you want to do this manually, set your view to camera view (Numpad 0) and enable view->Lock Camera to View. That will allow you to move the view to position it to a point where it closely matches the background image. Note that once you exit camera view or change to perspective view, your background image will no longer be displayed and or relevant. It will require some trial and error but it is possible, the trickiest part is going to be matching the camera lens...
Once you find the correct camera placement disable the lock camera to view option to prevent moving the camera. You might even want to lock all setting for your camera at that point.
Update for 2.8
With the camera selected, find the camera settings in the Properties window.
Enable Camera background.
Select the image.
The image will now show when using camera view.
NOTE: Blam is no longer mantained and has been replaced by Fspy: https://fspy.io/, but there is no importer for blender 2.79 or previous versions.
For a detailed post on how to use blam and Fspy read: How can I recreate geometry using a photograph?