Hi
Could,Will, Would: Asking people to do things (requests)(but could is more usual)
Examples; Could you wait a moment, please?
Liz, will you do me a favor?
Would you please be quiet? I'm trying to concentrate.
Could: Asking for things
Example; (during the meal) Could I have the salt, please?
Asking for and giving permissions
Example; 'Could I use your phone?' 'Yes, of course.'
Will: Offering to do things
Example; You look tired. I'll get you a cup of coffee.
Would: Offering and inviting
Example; 'Would you like a cup of coffee?' 'Yes, please.'
(Reference: English Grammar in Use; though it's book!)
Online Reference:
Would and Could: http://goo.gl/yoqeP
http://goo.gl/fBKfNj
Would, could and will: http://goo.gl/igXvuU