High Scalability Building bigger, faster, more reliable websites.

At least in the articles on Plenty of Fish and Slashdot it was mentioned that one can achieve higher performance by creating read-only and write-only databases where possible.
I have read the comments and tried unsuccessfully to find more information on the net about this. I still do not understand the concept. Can someone explain it in more detail, as well as recommend resources for further investigation? (Are there books written specifically about this technique?) I think it is a very important issue, because databases are oftentimes the bottleneck.