An affix refers to a morpheme combined to a root to add a lexical or grammatical piece of information.

There are usually 3 main types of affixes:

The prefix, added to the beginning of the word (e.g. un-happy)

The suffix, added at the end of the word (e.g. happi-ness)

The infix, added in the middle of the word.

There are also interfixes which are added between two morphemes and circumfixes, added on all sides of the root.