Microsoft's Common Language Runtime (CLR) is a series of subroutines that basically make up the foundation of a programming language. This is an integral part of Microsoft's .NET framework which is basically an extension of the operating system to provide additional functionality. And of course to tie the user to the Microsoft operating system.

In SQL Server 2005, CLR has been built into the database itself so that developers can approach problems by programming in either SQL or CLR or a combination of both. This book is a general introduction to programming in CLR with SQL Server 2005. It is intended to be an 'open book' that you would keep open in front of you as you program using the CLR functions.

These is some discussion in the book about the decision to use CLR or to stay with SQL, but obviously these authors are believers in CLR. If nothing else, reading the book will give you a good idea about CLR and whether you want ot use it or not.

Note that since the announcement of .NET, the developers of Linux, Oracle and others have announced that they will also offer .NET/CLR support. It appears this is going to be the way and the light in the future.