A glyph is a carved figure or character, incised or in relief; a carved pictograph; hence, a pictograph representing a form originally adopted for sculpture, whether carved or painted.
In typography, a glyph is a graphical representation of a character, sometimes several characters or only a part of character. A character is a textual unit whereas a glyph is a graphical unit. For example the sequence of ffi will be represented by one glyph in TeX.
In the simple case, for a given font (typeface and size), each character corresponds to a single glyph but this is not always the case, especially in a language with a large alphabet where one character may correspond to several glyphs or several characters to one glyph (a character encoding). The term is usually used in particular reference to outline fonts.