When used with SharePoint, Office Web Apps provide updated versions of Word Web App, Excel Web App, PowerPoint Web App, and OneNote Web App. All the three editions of SharePoint 2013 – Foundation, Standard and Enterprise – support
SharePoint 2013 Office Web Apps
as explained below:
View and edit with Office Web Apps in SharePoint 2010 and 2013
View Office Web Apps
By default, SharePoint Office Web Apps are view-only. View-only functionality is provided for free. This requires connection to Office Web Apps Server 2013 and an Office desktop suite license, such as Office Professional Plus or Office Standard. You will need to go in for SharePoint Farm plans with a minimum of 3 servers.
Edit Office Web Apps
If you are an Office 365 customer, then you can automatically create and edit Office files using Office Web Apps from an Internet browser. This requires connection to Office Web Apps Server 2013 and an Office desktop suite license, such as Office Professional Plus or Office Standard.
Office Web Apps Server Integration
For SharePoint Server 2013, Office Web Apps are now delivered in a new Office server product, Office Web Apps Server. A single Office Web Apps server farm can support users who access Office files through SharePoint Server 2013, Exchange Server 2013, shared folders, and websites. It is a simpler way of deploying and managing Office Web Apps SharePoint 2010 and 2013 in an on-premise environment.
To know more about the different plans that will suit you the best, read here.