Monsters are imaginary scary creatures that lurk in dark places and horror movies. If it's got 3 heads, shark teeth and lives under your bed, then it's probably a monster.
Monsters are big. Monsters are hairy. And monsters are certainly scary. Think of the Abominable Snow Man, that Loch Ness creature, or even Shrek — even though he's technically an ogre. This word isn't only for the imaginary, though: you could call anything freakish, frightening, or particularly evil a monster, like a cruel murderer or even a really big, violent storm.