Guides provide a way of measuring and dividing up a page for layout purposes, without directly impacting the look of the document; guides and grids wont print or be visible in the final product. Because frames and shapes easily snap to guides, setting them up early can insure that the final result has clean margins and consistent spacing. 

In the example below, the guides divide the page into sections with gutters of 1/4th inch, allowing for consistent and even spacing between columns of text, images and shapes. The size of the gutters will vary with the design of the project; if a little text needs to fill a big space, larger margins can help.

By default, InDesign measures in picas, a common unit of measurement for newspaper layouts; one pica is roughly 1/6th inch. Depending on the project, it may be easier to work with a different increment, such as inches or pixels. To change the unit of measurement, right click a ruler and make a selection.

Basic Guide controls:

When finished creating guides, navigate to View > Grids & Guides > Lock Guides. Locking the guides insure they are not accidentally selected, moved, or deleted while working on a project. Guides may be unlocked by following the same steps.