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: