On the SitePoint "Server Side Coding" blog, there's this look at caching to help solve some PHP performance issues.

In the good old days when building web sites was as easy as knocking up a few HTML pages, the delivery of a web page to a browser was a simple matter of having the web server fetch a file. [...] Then dynamic web pages came along and spoiled the party by introducing two problems: a delay for processing and a check of the file's age before it's sent to the browser.

They talk about what caching can do for you, what you can do (both server and client side) to prevent caching, working with the headers sent from and back out to the browser, using output buffering, partial caching and using PEAR::Cache_Lite in your application.