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caching means that the page is stored as html on the server and that is used for the duration of the cache lifetime, instead of the dynamic page wich makes database calls to the server on each page request. That is a lot faster, but it also means that the page will stay the same for the duration of the cache lifetime.
For pages that are very dynamic, set the cache lifetime to 0 (for forum, for example) or a low lifetime (30 secs, 1 minute). For less dynamic pages and blocks, use longer cache lifetimes.
Herko