An inflectional affix is an affix that:
* expresses a grammatical contrast that is obligatory for its stem's word class in some given grammatical context
* does not change the word class of its stem
* is typically located farther from its root than a derivational affix, and
* produces a predictable, non idiosyncratic change of meaning.
s: creates plural nouns, as in "She owns two dogs."
-s: creates possessive nouns, as in "I found my dog's leash."
-s: creates third-person singular form of verbs, as in "He walks."
-ed: creates past tense of verbs, as in "He walked."
-en: creates past-participle verbs, as in "I've given her a chance."
-ing: creates present-participle verbs, as in "I am running."
-er: creates comparative adjectives and adverbs, as in "She's smarter than I."
-est: creates superlative adjectives and adverbs, as in "She's the smartest of the bunch."
A derivational affix is an affix by means of which one word is formed (derived) from another. The derived word is often of a different word class from the original.
A derivational affix is not part of an obligatory set of affixes
* generally occurs closer to the root
* generally is more meaningful, and
* is more likely to result in a form that has a somewhat idiosyncratic meaning.