How does output caching work in ASP.NET?   

Output caching is a powerful method that enhances request/response throughput by caching the content generated from dynamic pages. Output caching is enabled by default, but output from any given response is not cached until explicit action is taken to make the response cacheable.

To make a response eligible for output caching, it must have a valid expiration/validation policy and public cache visibility. This can be completed using either the low-level Output Cache API or the high-level @ Output Cache directive. When output caching is enabled, an output cache entry is formed on the first GET request to the page. Subsequent GET or HEAD requests are served from the output cache entry unless the cached request expires.