are you a programmer? if not no need to ilsatnl it.If you are, MSDN (Microsoft Developer Network) as the word suggests is a developer centre where you can find Softwares developed by microsoft. They provide technical support and help developers to develop their client softwares using Microsoft developed softwares (visual studio,asp.net etc).Developers also can help develop the software to be more efficient.If you are a programmer you can subscribe to MSDN to get developer tool to enhance you skills.MS Windows SDK (software development kit) contains all tools, libraries, sample files to work upon, so that you can independently develop Applications for yourself hitherto for microsoft as well. you can develop applications for Operating system. for eg .their comes a SDK to develop Windows narrator under dot net framework.