Golems are men made of clay, soil, iron, and earth. Essentially, they are animated matter granted rudimentary intelligence. True golems were intelligent with their own will, but now are extinct, as of before Fablehaven. Golems are made to serve their master and are also known to be close to extinction.
Hugo is a Golem with intelligence. As the series progresses, with help from fairies, Hugo becomes bigger and smarter. By the end of the series, Hugo can talk and act on his own while still following the orders of his master(s). Because of this, Hugo is the last known, true Golem.
Dullions are Golems' crude cousins.