Microsoft .Net is our primary platform for software development. .Net Framework is a constantly evolving set of technologies and tools for various platforms and problems. For example, WPF for UI development, or Entity Framework for database access code.
.Net is used in such products as Dynamics CRM, Sharepoint, Skype, etc., and websites such as microsoft.com, stackoverflow.com, msn.com and many others.
Cross-platform applications can easily be built using .Net Framework. For instance, same application can easily be replicated to run on both Windows and Windows Phone, and even Android. Most .Net applications can be run on other operating systems.