Photoshop - Using Guides Let's start out by establishing that this tip is specific to Photoshop. It doesn't work the same way in any other image editors, so to see and use this you'll need to have Photoshop.
Guides are user-placeable lines that float over the entire image.
Objects you insert and the tools you use both snap to these guides at 8 pixels. Guides will save in the PSD format, however they don't print or export into other image formats. This is handy because you can have a visual cue of where things go and snap them to the lines and no one will ever know. You can move, remove or lock a guide to avoid accidentally moving it. You can pick the color you want them to appear as and the style, lines or dots.