Google Apps is a service from Google providing independently customizable versions of several Google products under a custom domain name. It features several Web Applications with similar functionality to traditional Office Suites, including Gmail, Google Groups, Google Calendar, Talk, Docs and Sites.
In addition to shared apps (calendar, docs, etc.), there is Google Apps Marketplace, which is an App “store” for Google Apps users. It contains various apps, both free and for a fee, which can be installed to customize the Google Apps experience for the user. Taken from: http://en.wikipedia.org/wiki/Google_Apps