A friar belongs to a religious order, a group within the Catholic church. A friar is similar to a monk.
Friars are like monks in that they are devoted to a religious life. The difference is that a friar lives and works among regular people in society, while a monk lives in a secluded, self-sufficient group of monks. The word friar developed in the thirteenth century from the Old French frere, "brother or friar." The Latin root is frater, or "brother."