The event system (see Events Extension Point) introduced in Content Services 7 enables out-of-process extensions. To support the event system, a REST API Java Wrapper is provided for easy development of extensions from Java applications. For more information, see REST API Java Wrapper.
You should get familiar with the Out-of-Process SDK as it is the recommended way of developing out-of-process extensions.
Read through the Getting Started to get up to speed on the difference between in-process and out-of-process extensions.
To fully understand the platform extension points it is a good idea to read Platform Architecture.