Virelay is the English version of the word, virelai, French. We cannot escape the French definitions of the word because the French of the 11th - 12th centuries created the form.

There are three or four versions: the virelai, the virelai nouveau, the virelay ancien, and a version I call virelai-ballad.

This is perhaps the most aggravating of forms, because it has so many definitions, ranging from those of the original to the newest version. However, that doesn't have to be a discouraging fact, because like most poetic forms, they each have their own endearing qualities. Perhaps the most endearing quality of the virelay for me is that one definition is closely related to the ballad.

Virelays are mostly written in tercets that rhyme aab,bbc. 

Yet, the modern virelay has a looser application because it can be written in stanzas or lines of indeterminate length and number: a spur of the moment example: aab bbcbbc ccd ddadda, or, a2a2b6, or a8a8b4. This list could go on and on, the only true requirement being that the last rhyme of any stanza (or tercet) becomes the same rhyme for the first line of the next, and that typically from stanza to stanza the lines that differ in rhyme also differ in length. Therefore in quatrains it would become a8b6a8b6, b8c6b8c6, c8d6c8d6, in cinquains it would become a8a8b6a8b6, or a8b6a8b6b6,...and so on, where the numbers here represent the number of syllables in the line. Those values could be switched, shortened or lengthened. There are some virelays that make the tercet the standard by making the rhyme scheme aabaab, bbcbbc, etc., and the examples that I've seen have very short lines between 2 and 6 syllables long.

But, there is just so much information about the virelay, that it is hard to know where to start. So, here a few links, to get you started:

I like these definitions the best, I think, because they refer to older resources: and I like the English example provided here:
http://en.wikipedia.org/wiki/Virelai

http://en.wikipedia.org/wiki/Virelai_ancien

http://en.wikipedia.org/wiki/Virelai_nouveau

Although it says otherwise, this is a newer version, the one I call the virelai-ballad:
http://www.poeticbyway.com/gl-uv.html#virelay
http://www.merriam-webster.com/dictionary/virelay

Hope this helps>>>don't be afraid to read all the postings, it'll give you a better handle on the origins.