Derivational affixes usually apply to words of one syntactic category and change them into words of another syntactic category. For example, the English derivational suffix -ly changes adjectives into adverbs (slow > slowly).
Some examples of English derivational suffixes:
Note that derivational affixes are bound morphemes. In that, derivation differs from compounding, where free morphemes are combined. It also differs from inflection in that inflection does not change a word's syntactic category and does not create new lexemes, but new word forms.
For other processes of word formation, cf. conversion and compounding.