There are two very clear definitions which I have found recently in the authoritative Answer.com.
· Free morphemes like town, dog can appear with other lexemes (as in town hall or dog house) or they can stand alone, i.e. "free".
· Bound morphemes (or affixes) like "un-" appear only together with other morphemes to form a lexeme. Bound morphemes in general tend to be prefixes and suffixes. Unproductive, non-affix morphemes that exist only in bound form are known as "cranberry" morphemes, from the "cran" in that very word.