Thread control block

Thread Control Block is a data structure in an operating system kernel that contains thread-specific information needed to manage the thread. The TCB is "the manifestation of a thread in an operating system."
Each thread has a thread control block. An operating system keeps track of the thread control blocks in kernel memory.
An example of information contained within a TCB is:

Thread Identifier: Unique id is assigned to every new thread
Stack pointer: Points to thread's stack in the process
Program counter: Points to the current program instruction of the thread
State of the thread
Thread's register values
Pointer to the Process control block of the process that the thread lives on

The Thread Control Block acts as a library of information about the threads in a system. Specific information is stored in the thread control block highlighting important information about each process.