A more sophisticated C# development environment. Think smart autocompletion, computer-assisted changes to source files, smart syntax highlighting and more.
VisualStudio C# 2010 is a product from Microsoft. It comes in an Express and a Profesional edition.
The Express edition is free, and you can download it from here.
The Professional edition is not free, you can find out more information about it here.
Unity’s VisualStudio integration allows you to create and maintain VisualStudio project files automatically. Also, VisualStudio will open when you double click on a script or on an error message in the Unity console.
Even though Visual Studio comes with its own C# compiler, and you can use it to check if you have errors in your c# scripts, Unity still uses its own C# compiler to compile your scripts. Using the Visual Studio compiler is still quite useful, because it means you don’t have to switch to Unity all the time to check if you have any errors or not.
Visual Studio’s C# compiler has some more features than Unity’s C# compiler currently has. This means that some code (especially newer c# features) will not give an error in Visual Studio but will give an error in Unity.
Unity automatically creates and maintains a Visual Studio .sln and .csproj file. Whenever somebody adds/renames/moves/deletes a file from within Unity, Unity regenerates the .sln and .csproj files. You can add files to your solution from Visual Studio as well. Unity will then import those new files, and the next time Unity creates the project files again, it will create them with this new file included.
Unity does not regenerate the Visual Studio project files after an AssetServer update, or a SVN update. You can manually ask Unity to regenerate the Visual Studio project files through the menu: Assets->Sync MonoDevelop Project