Do you think Sharepoint an engine for just document sharing and versioning? Oooooops..!!
A big no……It is a lot more than that. Apart from using InfoPath to make feeder forms for sharepoint web services, you can make it use as a platform for systems applications development.
.net Web Parts
To customize the sharepoint sites with user-defined business process flows and workflows, user should develop custom web parts on .net technologies. It is basically similar to an application development on .net with more or less constraints and standards.
For example, if the user wants the SP to display the logged in user name, he needs to write a web part to display it in the pages. There web part templates comes into picture. It is freely downloadable from Microsoft web site and it is a plug-in to the VS.Net 2003.