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Like other desktop environments, the Windows API heavily uses handles to represent objects in the system and to provide a communication pathway between the operating system and user space.
For example, a window on the desktop is represented by a handle of type (handle, window).
Process IDs and job IDs are explicitly visible integers, while file descriptors and sockets (which are often implemented as a form of file descriptor) are represented as integers, but are typically considered opaque.
In traditional implementations, file descriptors are indices into a (per-process) file descriptor table, thence a (system-wide) file table.
In non-capacities-based systems (as is more common), each process must acquire its own separate handle, by specifying the identity of the resource and the desired access rights (e.g., each process must open a file itself, by giving the filename and access mode).While a pointer contains the address of the item to which it refers, a handle is an abstraction of a reference which is managed externally; its opacity allows the referent to be relocated in memory by the system without invalidating the handle, which is impossible with pointers.The extra layer of indirection also increases the control that the managing system has over the operations performed on the referent.A resource handle can be an opaque identifier, in which case it is often an integer number (often an array index in an array or "table" that is used to manage that type of resource), or it can be a pointer that allows access to further information.Common resource handles are file descriptors, network sockets, database connections, process identifiers (PIDs), and job IDs.