Explain HTTP caching headers and their behavior.

Cache-Control: max-age (seconds fresh), no-cache (revalidate), no-store (don't cache), private/public, immutable. Expires: legacy, absolute date. ETag: resource version hash for conditional requests (If-None-Match). Last-Modified: timestamp for conditional (If-Modified-Since). Conditional requests return 304 Not Modified if unchanged. Vary header specifies which request headers affect caching. CDNs/browsers use these for cache decisions. Stale-while-revalidate serves stale while refreshing.