Application Request Routing (AAR) is an optional feature of IIS on Windows 2008+ that allows IIS to act as a reverse proxy and URL re-writer. With this support you can have an Windows 2008+ server in your DMZ, it does not have to be domain joined, that all external clients connect to, then it will proxy their request to Exchange.
So it basically is a light weight replacement for TMG, but it does not support advanced rules, pre-authentication, DoS detection, etc, etc.
Part 1: Reverse Proxy for Exchange Server 2013 using IIS ARR – Setting up a basic AAR config
Part 2: Reverse Proxy for Exchange Server 2013 using IIS ARR – Advanced setup, with service level availability and how to handle co-existence with Exchange 2007 & 2010
Part 3: Reverse Proxy for Exchange Server 2013 using IIS ARR – Tracing, Logging, & Best Practices