This post is written live from the SharePoint Conference 2012; it can be considered as an addition of my previous post about Office Web Apps 2013 (http://t.co/dfeg6HFX).
Office Web Apps allow to view and edit Office document with a web browser (no Office rich client needed neither used).
Office Web Apps are delivered through 3 channels:
- Consumer Cloud (SkyDrive)
- Public Cloud (Office 365)
- Private Cloud (On Premises)
Architecture
The same architecture is deployed whatever the channel used.
With previous Office Web Apps version, this was deployed within the SharePoint farm it self. With the new release, this is now split from SharePoint and can now be used by different SharePoint farms or other products, such as Exchange or Lync 2013 or also maybe any other third party products.
In Orange: Office Web Apps 2010 – In Green: Office Web Apps 2013
How Office Web Apps works
WOPI is Web Application Platform Interface is used to convert document to a web page to display the doc within the web browser (in the App Frame).
Office Web Apps requires dedicated servers (VM are supported) and can be deployed within a single server (but think about performance and redundancy). Regarding performance, you can expect about 20k typical users per server.
Once the Office Web Apps farm has been deployed, all serve farm are identical (exactly like SharePoint or Exchange [depending of the role deployed off course]). The farm can be reconfigure live, meaning no need to remove servers from the farm.
The configuration, as well as the connection with SharePoint farm, is done with PowerShell only.
The connection with SharePoint farm is done for all the farm for any of the SharePoint server farm.
Security Considerations
User credentials are passed on every request, while credentials are limited to permission for a single file.
If SharePoint 2013 is configured to used Office Web Apps, they both have a trusted connection.
Off course, like with SharePoint 2010 or 2013, you can restrict machines that can join a farm by settings the OU parameter.
Load Balancing
About any load balancer will work with Office Web Apps while front end affinity is highly recommended.
In case of SSL use, you should terminate your SSL at the load balancer level and not at the server farm.
Licensing
Office Web Apps are free 
, but only for viewing only.
If you need the editing feature, you need to licensed Office Web Apps.