https://technet.microsoft.com/en-us/library/jj219437.aspx
Office Web Apps Server is an Office server product that provides browser-based file viewing and editing services for Office files. Office Web Apps Server works with products and services that support WOPI, the Web app Open Platform Interface protocol. These products, known as hosts, include SharePoint 2013, Lync Server 2013, and Exchange Server 2013. An Office Web Apps Server farm can provide Office services to multiple on-premises hosts, and you can scale out the farm from one server to multiple servers as your organization’s needs grow. Although Office Web Apps Server requires dedicated servers that run no other server applications, you can install Office Web Apps Server on virtual machine instances instead.
It is easier to deploy and manage Office Web Apps within your organization now that it is a stand-alone product. If you deploy SharePoint 2013, for example, you no longer have to optimize the SharePoint infrastructure to support Office Web Apps, which in earlier versions was tightly integrated with SharePoint Server 2010. You can also apply updates to the Office Web Apps Server farm separately and at a different frequency than you update SharePoint, Exchange, or Lync Server. Having a stand-alone Office Web Apps Server farm also means that users can view or edit Office files that are stored outside SharePoint Server, such as those in shared folders or other websites. This functionality is provided by a feature known as Online Viewers.
The following illustration shows the differences between the previous deployment model for Office Web Apps and the new stand-alone deployment model for Office Web Apps.
The following Microsoft TechNet article describe the process required to deploy Office Web Apps Server Farm.
Deploy Office Web Apps Server
Configure Office Web Apps for SharePoint 2013
