Eric Lease Morgan has an article on SRU in the current issue. Ralph Levan gets a namecheck (Ralph's SRU code here).
This article is an introduction to the "brother and sister" Web Service protocols named Search/Retrieve Web Service (SRW) and Search/Retrieve URL Service (SRU) with an emphasis on the later. More specifically, the article outlines the problems SRW/U are intended to solve, the similarities and differences between SRW and SRU, the complimentary nature of the protocols with OAI-PMH, and how SRU is being employed in a sponsored NSF (National Science Foundation) grant called OCKHAM to facilitate an alerting service. The article is seasoned with a bit of XML and Perl code to illustrate the points. The canonical home page describing SRW/U  is also a useful starting point.[Main Articles: 'An Introduction to the Search/Retrieve URL Service (SRU)', Ariadne Issue 40]