Clarification, Google is a search engine and Google Chrome is the browser which Google can use. However other browsers can also be used or which are able to support the Google atmosphere. So which are you referring to.. Google, Gmail, Google+, Google Chrome or all of the above?..
Google Now is a front-end for Google searching, to an extent. There are some advantages to using it on a smartphone, integration with the apps (like email, text, etc) but generally what happens when someone says "OK Google" is a search of the web.