Visual Studio LightSwitch is a separate version of Visual Studio which in the VS product hierarchy sits between the free Express series and Visual Studio Professional.
LightSwitch is focused on rapid development data centric applications which are not intended for high scale or load. So a typical LightSwitch app would be a simple invoicing application for a small company that needed to issue and track invoices. LightSwitch apps are in general not intended for large numbers of users.
LightSwitch applications are either desktop based Silverlight oWPF apps, or alternatively can be deployed remotely on as a website or on Windows Azure.