This is one of Photshop's best features. Page layout is now much easier in Photoshop and you no longer need to resort to Illustrator to make those geometric shapes and then import them.
Guides appear as lines floating over the image.
Once you place a guide, you can move it, remove it, or lock it into place. Selections, and tools snap to the guide or to the grid when they are dragged within 8 screen pixels of it. Guides will also snap to the grid when moved. Snap To Grid. How To Use Them Turn guides on by first turning the rulers on.