The Common Language Runtime (CLR) is the foundation upon which the Microsoft® .NET strategy is built. The CLR provides an execution environment that manages running code and provides services that make software development easier. These services include automatic memory management, cross-language integration, interoperability with existing code and systems, simplified deployment, and a fine grained security system. The .NET platform also brings with it a new programming language called C#. It is very similar to Java and allows developers to make full use of all features available on the .NET runtime.
The CLR can run a variety of different types of applications, such as: console applications, Web server scripts, traditional Win32-based applications, and more.
The CLR provides a language specification for compilers to generate classes and code that can interoperate with other programming languages (The Common Language Specification: CLS) . Any API that is written using a CLS provider language can be used by any language that is a CLS consumer. Compilers generate code in a format called Common Intermediate Language (CIL) which is an intermediate representation of a compiled program and is easy to compile to native code or be compiled using Just-in-Time (JIT) engines. The restrictions placed by the runtime on the CIL byte codes ensures good potential for code optimization by the JIT compiler. The runtime environment provides garbage collection, threading and a comprehensive class library. All these things make code development easier and more appealing by allowing a mixture of languages to be collaborating to produce results, all contained within a managed sandbox to prevent bugs from breaking other running programs.
Virtuoso is a CLR host. It is responsible for initializing the runtime, defining the application domain (its runtime boundaries, security settings, etc), and executing user code within such domains. Windows can be used as the development platform, a very developer friendly environment with a rich set of tools, but you will not be restricted to Windows, .NET and IIS to run the assemblies produced. Where .NET is not readily available or desired Virtuoso contains Mono, an open source re-implementation of Microsoft .NET, a vehicle making .NET cross-platform.
The CLR run time is a part of the Virtuoso process. The Mono run time may be run either as in-process or as out-of-process. Hosted applications can make use of the regular Virtuoso .net data provider to access Virtuoso SQL data and procedures. Microsoft ASPX files can also be run directly from Virtuoso either from the filesystem or WebDAV. Each of these capabilities releases you from the Microsoft platforms without compromising your development platform.