Guides are very different things: these are helpers which give you an easy way to place objects where you like.
Guides can be placed manually by pressing the mouse button in a ruler and dragging them onto the page. The guide is placed when the mouse button is released. It can be moved afterwards with the Select Item tool (C key).
Tip
Can't see the guides!
If you don't see the guides, select View | Show Guides or verify that the display is not in Preview Mode (in the View menu).
If a guide is placed in a master page, it will appear on each page on which the master page will be applied. It is really useful if you want to place a Text Frame or Image Frame at the same place on several pages.
The difficult part of this is that it is difficult to place a guide very precisely, for example, at the middle of the page. The Page | Manage Guides window helps you position guides according to a precise value (Single tab) or create several well distributed (Column/Row tab) guides at once.