A sandbox is a restricted execution environment that enables programs to access only certain resources in a controlled way
A sandbox is a restricted execution environment that enables programs to access only certain resources, and that keeps problems that occur in the sandbox from affecting the rest of the server environment. Solutions that you deploy into a sandbox, which are known as sandboxed solutions, cannot use certain computer and network resources, and cannot access content outside the site collection they are deployed in.
Questions having the sandbox solution tag refers to the development and implementation of sandbox solutions.
Reference: http://technet.microsoft.com/en-us/library/ee721992.aspx
