You can use this dialog to add or remove guides. These can be used to conveniently align objects and create consistent layouts. The dialog looks like this:
You can have an arbitrary number of vertical and horizontal guides. Note that guides are defined on a per-page basis, i.e. you can have different guides on different pages. If you want to have the same guides on all pages, you can use the button "Copy to all pages" to copy the current guides to all pages.
Guide colors can be configured in the preferences dialog. See Preferences dialog for details.