In Continuation with Developing SharePoint Sandbox solutions Tutorial Part 1, Next lets look at how does Microsoft restrict Sandbox solutions of using various APIs.
Every server on which the SharePoint Foundation Sandboxed Code Service is running, there is a sandboxed worker process in which sandboxed solutions are executed. Calls from code in the sandboxed worker process to the Microsoft.SharePoint.dll are redirected to a special version of that assembly. The special version is in some ways less privileged than other assemblies in the sandboxed worker process.
The most significant restriction is that the special version contains only a subset of the standard assembly’s classes and methods. An exception is thrown when code calls an API that is not included in the special version of the assembly. In addition, even when a method is included in the special version of the assembly, it may impose additional restrictions on the parameters passed to the API. For example, the SPSite(String) and SPSite(Guid) constructors are available to sandboxed solutions, but only URLs or GUIDs that refer to the site collection in which the sandboxed solution is installed can be passed to them. This prevents a sandboxed solution from accessing any websites, lists, or other resources from outside the site collection to which it has been installed.
Now Lets look at what all you can create using Sandbox solutions (MSDN examples)