Over at WebmasterWorld Tedster references an interesting short paper about creating your own search engine by Googler Anna Lynn Patterson. This document makes for a good read.
The paper was published in April of 2004 when she was a student at Stanford University. She is also the person whose name appears on the recent Google patent application titled Detecting spam documents in a phrase based information retrieval system.
Basically, she breaks it down into hard drive space, having lots of servers, and CPU power. Anna's document is a good initial primer, but there is another aspect of building a search engine that deserves some emphasis.
The search engine companies have built the largest networks of servers the world has ever known. When I think of Google's core technology assets, I don't think about search engine algorithms, I think about massively deployed server networks operating in close harmony.
Optimising Digital Marketing Campaigns with Search, Social and Analytics
At SES London (9-11 Feb) you'll get an overview of the latest tools, tips, and tactics in Paid, Owned, Earned, Integrated Media and Business Intelligence to streamline your marketing campaigns in 2015. Register by 31 October to take advantage of Early Bird Rates.