Guides and the grid help you position images or elements precisely. Guides appear as nonprinting lines that float over the image. You can move and remove guides. You can also lock them so that you don't move them by accident.
The grid is useful for laying out elements symmetrically. The grid appears by default as nonprinting lines but can also be displayed as dots.
Guides and grids behave in similar ways:
You can use Smart Guides to help align shapes, slices, and selections. They appear automatically when you draw a shape, or create a selection or slide. You can hide Smart Guides if you need to.