When Microsoft first released InfoPath, in 2003, the software had some serious limitations. There was no easy way to combine InfoPath with programming languages. As a result, there was no easy way to use a language to refine the functionality of a form that had been designed with InfoPath.
When Microsoft launched the Visual Studio 2005 edition, they also released a toolkit for InfoPath that allowed programmers to integrate Visual Studio and InfoPath, in order to create managed code solutions for Microsoft Office applications. Managed code is program code that executes under the supervision of a virtual environment. Programs in any language can be compiled either into managed code or unmanaged code. Here, the code built on a .NET framework is meant to be managed code.
The bottom line is that VSTO 3.0 is well-integrated with Visual Studio 2008. It provides .NET platform developers with tools for building applications that influence Microsoft Office InfoPath...