Page Cache help

It is important to specify one of Expires or Cache-Control max-age, and one of Last-Modified or ETag, for all cacheable resources. It is redundant to specify both Expires and Cache-Control: max-age, or to specify both Last-Modified and ETag.


Expires and max-age do the same thing, but in two different ways. Same thing with Last-Modified and Etag


should I do expires or max-age?

Expires depends on accuracy of user's clock, so it's mostly a bad choice (as most browsers support HTTP/1.1). Use max-age, to tell the browser that the file is good for that many seconds. For example, a 1 day cache would be:
Cache-Control: max-age=86400


If I have to also do Last-Modified or ETag, which one of those? I think I get Last-Modified

You're right, Last-Modified should be better. Although it's a time, it's sent by the server. Hence no issue with user's clock. The browser sends the Last-Modified the server sent last time it asked for the file, and if it's the same, the server answsers with an empty response «304 Not Modified»


which files should I enable browser caching for?

All files can benefit caching. You've got two different approaches:

with max-age: useful for files that never change (images, CSS, javascript). For as long as the max-age value, the browser won't send any request to the server. So you'll see the page loading really fast on the second load. If you need to update files, just append a question mark, and the date of change (for example image.png?20110602). This way you can make files expire if it's urgent (remember that the browser almost never hits the server once it has a max-age file). Main use is to speed things up and limit requests sent to the server.
with Last-Modified: can be set on all files (including those with max-age). Even if you have dynamic pages, you may not change the content of the file for a while (even if it's 10 min), so that could be useful. The main use here is to tell the browser «keep asking me for this file, if it's new, I'll send you the new one». So, there's a request sent on each page load, but the answer is empty if the file is already good (304 Not Modified), so you save on bandwidth.
The more you cache, the faster your pages will show up. But it's a difficult task to flush caches, so use with care.

A good place to learn all this with many explanations: http://www.mnot.net/cache_docs/