Misconception of data warehouse?

I used to think data warehouse to be a VLDB. I then started asking some DBA's/Developers. To my surprise, 4 out of 5 people whom I expected them to know what a data warehouse told me what I thought it was at the first place. So, what is data warehouse? How is it implemented ? Why use data warehouse ? The above are some of the questions that I would like to discuss. If you have further comments or are board of reading my posts please let me know and I will promise will stop ?