If from the etymological point of view the term category denotes a class of objects sharing a common feature, under linguistic category we generally understand a class of linguistic units (chiefly words) sharing a common meaning or syntactical function, expressed by a common external (phonetic) form. Thus the words room-s, table-s, pencil-s, match-es, glass-es… are representative of the category of the plural number; (be) describe-d, contrive-d, share-d, found-ed, assert-ed… represent the category of the past tense. Forms like men, children, etc., on the one hand, (he) rode, went, etc., on the other, are also members of the category of the plural or past, respectively, in spite of the lack of the characteristic feature -(e)s or -(e)d, since functionally (i.e. as regards meaning) they lean upon the pattern of the productive -s and -d forms, thus:

the semantic difference between man and men (child and children) is the same as that between e.g. room and room-s, rendered by the sign -s versus zero;

the semantic difference between rides and rode (goes and went) is the same as that between describe-s and describe-d, rendered by the sign -d as against -s.