Affixes are linguistic elements added to a base word to modify its meaning or grammatical function. They include prefixes, which are added to the beginning of a word (e.g., "un-" in "undo"), and suffixes, which are added to the end (e.g., "-ing" in "running"). Additionally, infixes and circumfixes exist in some languages, but prefixes and suffixes are the most common in English. Affixes play a crucial role in word formation and can change a word's tense, aspect, mood, or meaning.