I know the topic has been covered here in the past...however I found this easy-to-read-and-sometimes-even-humorous article..... a good introduction to the subject
"
If alder is the hottest, most shallow drunk chick at the party, ash is her best friend. You know, the one taking a footing behind her friend, not wanting to cause such large ripples. The one with her own intellectual quirks that drive you wild." ?
http://audio-pro-central.com/WordPress/?p=40
Also another more detailed article from the desk of Alan:
http://www.ratcliffe.co.za/articles/bodywoods.shtml