I'm pretty sure the word you're looking for is inflected, meaning it experesses its grammar through modifications to the stem (not always as a change in a suffix or prefix, sometimes as an actual change to the stem itself, as is the case with the West Germanic Strong Verbs, incorrectly called 'irregular' by instructors of modern English). Though one can speak of different degrees of inflection in language, English is weakly inflected (or modern and middle English, anyway, Old English was moderately inflected), as is the case with Sweedish, Norwegian, Danish, Afrikaans, the Romance Languages are somewhere inbetween weakly and moderately inflected, German and Icelandic moderately so, Latin rather heavily inflectional, but Basque puts them all to shame.
Or, perhaps (though less likely so), the word you may be getting at is 'agglutinative', these are languages which do not so much change their lexeme for gramatical purposes (as is the case in inflected languages) as combine different lexeme togther in the form of prefixes and suffixes for the purposes of gramatical constructions. The Latin derived 'antidisestablishmentarianism' is a clear cut Modern English example of an agglutination, yet the class 4 Germanic strong verb 'break, broke, broken' is a clear cut Modern English example of inflection. This is extremly common in the Finno-Ugric and Samoyedic language groups; however, these languages will often be labeled as inflected not so much because of a confusion in terms as because of the evolution of these languages, especially in the Samoyedic branch of the Uralic languages. Specifically, consonant gradation is changing and merging the roots and suffixes, removing distinctions between the two, thus making elements of the langauges more inflected than agglutinative.