着る vs 着ている (put on vs wear)

The verb 着る in plain past expresses the act of putting on clothing (perfective); the ~ている form expresses the resulting state of wearing it (resultative). The contrast between perfective and resultative readings runs through many Japanese clothing/state verbs — 知る/知っている (come to know / know), 結婚する/結婚している (get married / be married).