Artisans are very skilled people usually in only one profession usually they make things by hand, so in medieval times this could be someone like a stained glass window maker or blacksmith.
A Merchant is basically a trader, they sell things.
A serf is a very low class farmer, a class of peasant
The role of the serf is to serve the nobles, merchants, and ruling classes. The ruling classes well, ruled, and the nobles were their vassals. The artisans and merchants sold goods.
