Varnish Cache is a web request accelerator, sometimes referred to as a HTTP accelerator, that will significantly enhance your web performance.
Varnish speeds up your store by storing a copy of the page in RAM memory the first time a user visits that page. Varnish can also be configured to cache only some bits and pieces of a page, which means that your store will remain as dynamic as it was.
What all this means is that with Varnish your store will be able to handle much more traffic and your store performance and scalability will be supercharged.
