A glaze is just a translucent color + a lot of medium (I use Liquin). This is "floated" over a dry surface and it will gently alter the color beneath. Please note that several thin layers are preferable to one thick one.
For example, some translucent colors I glaze with are burnt sienna, raw umber, prussian blue, alizarin crimson, (etc.)....as opposed to heavily pigmented and opaque colors like yellow ochre, Indian red and titanium white.
Here is another post on underpainting:
http://forum.portraitartist.com/show...=&threadid=190
Here is a post on the list of colors I often use to glaze with:
http://forum.portraitartist.com/show...?&threadid=210