What is a 'Cloud' (e.g. private cloud, computing cloud)

[Guy]

What is cloud computing? a private cloud ? and a public cloud??
the reason why i am asking is because i'm thinking of buying a new macbook and use this laptop as a server for my use (put my website on, and maybe my clients websites.. but not a website hosting service.)
and i saw ubuntu server...
and on this page...
http://www.ubuntu.com/cloud (ubuntu server edition)

thanks!