Golems are magically created monsters. There are four different sorts,
<There are four different sorts of greater golems, and, four different sorts of lesser golems: each will be detailed separately.>
and each will be detailed separately.

All but the flesh golem are created from earthen components.
The former is created from the remains of humans.
The creation of a <greater> golem involves ultra-powerful spells and elemental forces.

Magical creatures with HD equal to or greater than those of the golem
are able to attack with effect. For purposes of determination of HD,
figure 4.5 HP per HD, and points being rounded up.
Thus

SA: Decapitating golems


 

Q: The Monster Manual says that
golems can be hit by magical creatures
that have as many as or more
hit dice than the golem. What is a
magical creature?

A: This is a limitation on the Creatures
Struck Only By Magic Weapons table
found on page 75 of the DMG. In order to
affect a golem, a creature must have an
attack equal to a magic weapon of the
appropriate bonus and have hit dice equal
to or greater than the golem's. For example,
a flesh golem is struck only by +1 or
better magical weapons. A werewolf,
being itself struck only by magical weapons,
is the equivalent of a +1 weapon
according to the DMG table. However, it
has only 4 + 3 HD and the golem effectively
has 9, so the werewolf cannot harm
the golem.
(131.63)