Keyvan Nayyeri knows Visual Studio Extensibility all right. He even wrote a book:

Now he's written a multi-part blog series about extending Visual Studio.

• Part 1 clarifies the difference between an add-in and a package, and acknowledges that there are also other ways of extending Visual Studio (it's actually incredibly extensible in a lot of different ways). It's a nice history review to show us how we got to where we are.
  
• Part 2 is about the IDTExtensibility2 and IDTCommandTarget interfaces. All add-ins implement the first, and those that are on the Tools menu implement the second.
  
• Part 3 turns to packages, which inherit from the abstract class Package (and therefore have plenty of work to do implementing various interfaces.) Keyvan recommends C++ for package development, interestingly enough.
• Part 4 can now talk about the differences between them in some detail. Add-ins are easier, can be written in more languages, but just can't get the same level of integration that a package can. It depends on the functionality you plan to offer your users.
  

Looks like there are more parts to  come ... stay tuned!

Kate