A glyph is the particular look of a 
character.  Note that this is extremely specific; the glyph "a" is different from "
a" and both are different from "
a".  Virtually everyone insitinctively abstracts away from glyphs to characters, so the only people who are usually aware of them are people working with fonts (or doing advanced type-setting, or anything else like that.)
A glyph may be implemented as a bitmap (mostly for low resolution) or as vector graphics describing the outlines. Very simple bitmap example, from 
strimj - string image routines:
       "...@
        ..@@@
        .@@@@@
        @@@@@@@
        ...@
        ...@
        ...@
        @@@@@@@
        .@@@@@
        ..@@@
        ...@"