Working on complex project usually requires adding loads of libraries (dependencies or packages). Those libraries require other libraries which require other libraries etc. So that led to creation of package management systems or package managers such as NuGet for .NET Framework) or Maven for Java. List of all package managers can be found here.
NuGet package manager is available in Visual Studio 2012 in Project -> Manage NuGet Packages…
There you can see packages installed in current project, as well as packages which are available online, along with package info.
To install package, select it and click “Install”, and that is going to install the newest version of selected package.
If you need older (specified) version of certain package, you can install it from Package Manager Console (Tools -> Library Package Manager -> Package Manager Console). After that you have “PM>” prompt for typing commands for package manager.
Command for installing for example Version 5.0.0 of EntityFramework is :
Install-Package EntityFramework -Version 5.0.0
References
http://en.wikipedia.org/wiki/NuGet
http://en.wikipedia.org/wiki/Apache_Maven
http://stackoverflow.com/questions/5628689/download-old-version-of-package-with-nuget