NGEN, the Native Image Generator has been part of the .NET Framework for several years, creating optimized native images of managed code applications which are then used at runtime. NGEN includes a component that runs as a background service as it attempts to ensure .NET applications are consistently optimized.
The latest evolution, AutoNGEN, arrived with Windows 8 and has been further refined for Windows 8.1. With Windows 8, NGEN was designed to only optimize .NET apps or libraries that are actively being used on the machine. In Windows 8.1 this service seeks to take advantage of Microsoft’s build systems so that .NET-based packages from Microsoft are optimized by a NGEN-like service prior to being made available for distribution.