Poetry: alliteration

I'd like to know what alliterative verse is. I have a
nebulous idea of sounds being echoed throughout a
poem, but what are the rules? When does this 'echo'
stop being a simple poetic device to become a genre?

All this is because of an alliterative poem I read, in
Tolkien's _Unfinished Tales_, 'The Istari':

  Wilt thou learn the lore / that was long secret
  of the Five that came / from a far country?

etc. If any of you can give another example, I'd be grateful.
I understand that OE poetry was based on alliteration, am
I right? I'm trying to produce some of this in Draseléq, since
sheer metrics and/or rhyme don't favour my muse...


--Pablo Flores
  http://www.geocities.com/pablo-david/index.html
  http://www.geocities.com/pablo-david/draseleq.html