Here's an introduction by researchers at Middlebury with a discussion about its applicability to search:
Precision, Ranking, and Recall - the Holy Trinity
In talking about search engines and how to improve them, it helps to remember what distinguishes a useful search from a fruitless one. To be truly useful, there are generally three things we want from a search engine:
We want it to give us all of the relevant information available on our topic.
We want it to give us only information that is relevant to our search
We want the information ordered in some meaningful way, so that we see the most relevant results first.
