SharePoint 2010 provides a new sandboxed environment that enables you to run user solutions without affecting the rest of the SharePoint farm. This environment means that users can upload their own custom solutions without requiring intervention from administrators, and without putting the rest of the farm at risk. This means that the existing sites\pages or components will not be effected by the newly added soultion. Users can deploy the below four things as sandboxed soultions :
2. Event Receivers.
3. List Definations.