Guild Masters can be found in every town where they are able to provide several services for the player.
Guild Masters could be thought of as the Village Elders of towns.
Although any townsperson can tell you where the Guild Master is through conversation, they do not actually give useful directions, merely stating he is "over there". This means you will have to hunt them down yourself. Just look for someone standing still who is also not in armor or next to a stall of merchandise.
You can speak with the Guild Master concerning several topics: