What is a glyph and what do we use it for especially in context of silverlight and xaml?
Glyphs are a low-level depiction of a character to be drawn on-screen. A good article on them is available on MSDN: Introduction to the GlyphRun Object and Glyphs Element.
Posting a link for you read, I think it is better than simply rehashing the information in my answer (it has pictures and links for further reading).