Names have always been powerful things. In the Otherworld, to know the name of something is often to know the thing itself. Creatures of the Otherworld seldom divulge their true names, and will go by a euphemism or nickname.
However just because you know something doesn't mean that you automatically have power over it. Many things in the Otherworld have ways of finding out who is talking about them, and it's dangerous to invoke something that you have no way to confront.
In particular, the number of euphemisms used to refer to the Fae reflects how dangerous it is to inadvertently attract their attention. Many of the names are complementary or ironically polite, reflecting the terror which those who used them had of offending these mischievous and powerful beings.
Some of the more common terms:
The original document is available at http://www.thereamid.com/tiki/Euphemisms