An affix is a morpheme that is attached to a word stem to form a new word. Affixes may be derivational, like English -ness and pre-, or inflectional, like English plural -s and past tense -ed. They are bound morphemes by definition; prefixes and suffixes may be separable affixes. Affixation is, thus, the linguistic process speakers use to form different words by adding morphemes (affixes) at the beginning (prefixation), the middle (infixation) or the end (suffixation) of words.
Read more about Affix: Positional Categories of Affixes, Lexical Affixes, Orthographic Affixes