Microsoft Knowledge Base Article 137983 contains the following summary:
When a Microsoft SQL Server client disconnects from a SQL Server server, the connection process should be cleared up on the server side. If the connection processes are not cleared up for any reason, they become "orphaned" or "ghost" processes. These processes may use up valuable resources such as locks and user connections. The orphaned processes are typically caused by improper closing of client applications and network-related problems, and the remedies usually require troubleshooting client applications and fine- tuning network configurations.