When you've to develop a custom solution based on WSS/MOSS, I see that sometimes there are doubts on what technology to use in order to interop with the Sharepoint platform: should I use the Sharepoint Object Model APIs or should I use Sharepoint Web Services?
Use the Sharepoint Object Model if your code will be executed on a Sharepoint server farm machine. This is the best way if the final application will run on the Sharepoint server or if you're developing a Sharepoint WebPart.
To work with the Sharepoint Object Model, Right-click your Visual Studio project and add a reference to the Microsoft.SharePoint.dll located on the following directory:
C:\program files\common files\microsoft shared\web server extensions\12\isapi
If you're planning to develop a custom application that needs to interact with Sharepoint from a remote client, the best way is to use Sharepoint Web Services (the object model is not designed to support Remoting).
