VSTA

VSTA stands for Visual Studio Tools for Applications and is a set of tools that independent software vendors can use to build customisation capabilities into their applications.
VSTA will have an IDE similar to Visual Studio and will continue to be shipped with the Microsoft Office applications.

Office 2007 continue to come with VBA as the end-user customisation environment except for InfoPath 2007 which allows you to customise the forms by programming with Visual Studio for Applications.

What are the benefits of VSTA ?

1) An Excel application written in VSTA can be easily converted to a Visual Studio project and then distributed as an application.
2) Contains built-in versioning
3) VSTA can use the .NET Framework
4) There is still macro recording in Excel and Word ?
5) Code is not saved in the actual file but in separate assemblies
6) Code is isolated and located into separate application domains

Visual Studio 2005 Tools for Applications

VSTA can exists side by side with VBA in the host application.
This will allow the host to slowly migrate to managed code.

The main component of the VSTA is the "AddInManager" which is a group of classes that are used to manag add-ins.
Insteadm the AddInManager consists of several discreet classes.
The most basic of these are the:
AddIn
AddInCollection
Context
All of these are packaged in an assembly named Microsoft.VisualStudio.Tools.Applications.AddInManager.dll
and are in the Microsoft.VisualStudio.Tools.Applications namespace.
There are many classes in the AddInManager, mostly used in more advanced scenarios.

So what are each of these classes ?

AddIn Class

This represents an assembly containing entry-points that compose a user-created add-in.
In most cases the assembly will be the output of the VSTA project that was built by the end-user in either VSTA ot Visual Studio IDE
This is probably the most important class in the AddIn Manager.

AddInCollection Class

This a collection of AddIns

Context

Used to define behaviours that are common to all the add-ins it contains, such as the HostItemProvider and the Evidence used if any AppDomains need to be created.