You can select the View > Add Horizontal (or Vertical) Guide menu item from the View menu or by control clicking to add a Guide to your image. You can use this guide visually for laying out graphics and you can also snap vectors to them.
You can drag guides to move them and drag them out of the image to remove them. You can also double click them to convert them between vertical and horizontal guides.
You can also edit the color for guides in the Aligning Preferences.