Orc vs Ork: What's the difference?

I'm hoping for clarification on some fantasy-based terminology. I've read a number of websites that refer both to "orcs" and"orks", occasionally in the same sentence! Previously, I had only ever seen the term "orc" in fantasy writing.

From what limited information on the topic I've been able to find, "orks" seems to be a term for an orc-like creature in the Warhammer 40K series.

What I want to know is does the term "ork" apply exclusively to the character in the Warhammer 40K universe or does it also represent something else? Can "ork" and "orc" be used interchangeably? If does only apply to Warhammer, should it be considered a proper noun and capitalized at all times?

Ork is also a creature from European legend. If you can speak German the original link is here however a rough translation is as follows:

It is believed that in the early Middle Ages, due to the advent of Christianity, the god Orcus was villified as an evil devil-like creature.

In Austria the Ork lives on mountaintops and in forests and is a solitary creature; lord of the forests and animals. He warns animals when the hunters come and can encourage wild animals to steal cattle, thus ruining livelihoods. Occasionally he's believed to be a shapeshifter.

Variants of Orks include Norks who are house-spirits that live in cellars and take the form of a grumpy goblin.

The words orc, ork and ogre stem from the same place. Orcus was originally a god of the underworld; a punisher. The belief in Orcus was particularly prevalent around Tarquinia where it is thought he took the form of a large bearded giant.

As an addendum to Ilythya and njboot's answers, there is one form of Ork that is unrelated to orc.

L. Frank Baum created the creature called an Ork in his Oz universe. It is a gentle, flying creature that seems completely unrelated to the Warhammer 40K creature, or the creature from Germanic legend.

You can read more at Google books.

According to Wikipedia, Ork only refers to Warhammer 40000:

"Ork (Warhammer 40,000), a fictional species in the Warhammer 40,000 universe"

However, the "Ork" in Warhammer 40000 still constitutes an orc, which is defined as:

noun (in fantasy literature and games) a member of an imaginary race of humanlike creatures, characterized as ugly, warlike, and malevolent.

(Source: New Oxford American Dictionary).

You can think of it in terms of "a square is a rectangle, but not all rectangles are squares." An "Ork" is considered an "orc," however not all "orcs" are "Orks."