When you deploy a Web Part to SharePoint, you must first make it as a safe control to use within SharePoint in the web.config file. If the safe control entry exists, the object will be loaded otherwise they will throw an error.
How to add the safe control from Visual Studio…
What is the GAC?
The GAC stands for the global assembly cache. It is the machine wide code cache which will give custom binaries place into the full trust code group for SharePoint. Certain SharePoint assets, such as Feature Receivers need full trust to run correctly, and therefore are put into the GAC.
More info on deployments and what it means to have your code in the GAC…
http://technet.microsoft.com/en-us/library/cc261736.aspx
