If I understand your question correctly, you want to know how fonts are defined in u8glib?
U8glib has its own format for fonts. It is compact and partly compressed to allocate as less of flash rom as possible.
All u8glib fonts are derived from the bdf font format (Glyph Bitmap Distribution Format - Wikipedia)
A tool (bdf2u8g) can convert a bdf file to the internal format: Google Code Archive - Long-term storage for Google Code Project Hosting.. This c file also contains the description of the internal format.
Hope this answers your question.
Oliver