It is a tool which built applications which are built on classic three-tier architecture, on top of existing .NET technologies.
It is a combination of GUI Layer + Business Logic Layer + DB Layer
GUI - Silver light, which workd for windows or web.
Business Logic Layer - ASP.net WCF service or OData
DB Layer - SQL Server, SQL Azure, Sharepoint
According to the Microsoft
"The LightSwitch presentation tier is a Silverlight application. It can run as a Windows desktop application or hosted in a browser. The LightSwitch logic tier exposes a set of WCF RIA DomainServices running in ASP.NET. The logic tier process can be hosted locally (on the user’s machine), on an IIS server, or in Windows Azure. A LightSwitch application’s primary application storage uses SQL Server or SQL Azure and can consume data from existing SharePoint 2010 lists, databases accessible via an Entity Framework provider, and custom build WCF RIA DomainServices.
With LightSwitch, your application can point to an existing data source—or databases that are automatically created—without the need to write code. For the user interface, LightSwitch provides screens that are based on predefined templates. They allow you to display data automatically by simply specifying which elements you want to show.
For the business logic, LightSwitch allows you to do simple validation such as required fields and minimum numeric values. But LightSwitch also gives developers the ability to write more complex business logic code for such scenarios as saving changes to a data source or performing authorization operations."