I think they should have written a bit more in the PHB about creature types, at least because of the Ranger's Favored Enemy ability.
Anyway, "humanoid" are creatures with physical and mental features very close to humans', that is a torso, two arms, two legs, a head, and a comparable size; they are also basically without any spell-like or supernatural ability (although Gnomes' cantrips for example seems to be an exception).
Your DM can tell you the whole list of humanoids (they're about a dozen in MM), which is basically made up of Humans, demi-humans (the other PC races from PHB), Goblinoids and Repltilian humanoids, plus some relevant extras (Orcs, Gnolls...).
Also note that Giants are Feys are NOT humanoid, although they may seem at the sight.