A service from Google providing many of their online services with a custom domain name
Google Apps is a service from Google that provides individuals and businesses customizable versions of many of Google's online services using their own domain name.
Google Apps was launched in August 2006 and has expanded expanded its services over time largely due to the sheer demand for custom domain emails using the Gmail service, as well as other useful applications like Google Calender, Drive and Hangouts.
Google is constantly adding services to Google Apps as demand increases. Many of their Google App services are free for small business.
You can read more about Google Apps by visiting their website.