A command queue is a queue for delaying the execution of commands, usually either in order of priority or on a first-in first-out basis. They are often useful in synchronous

Synchronization (computer science)

In computer science, synchronization refers to one of two distinct but related concepts: synchronization of processes, and synchronization of data. Process synchronization refers to the idea that multiple processes are to join up or handshake at a certain point, so as to reach an agreement or...

 applications, where a command executor may receive a new command while it is still performing a previous one, and so requires a means of tracking what the commands are, and in what order they must be performed.

Example:

• Native Command Queuing
  Native Command Queuing
  Native Command Queuing is a technology designed to increase performance of SATA hard disks under certain conditions by allowing the individual hard disk to internally optimize the order in which received read and write commands are executed...
  
   (NCQ) in Serial ATA
  Serial ATA
  Serial ATA is a computer bus interface for connecting host bus adapters to mass storage devices such as hard disk drives and optical drives...
  
   (SATA)
• Tagged Command Queuing
  Tagged Command Queuing
  Tagged Command Queuing is a technology built into certain ATA and SCSI hard drives. It allows the operating system to send multiple read and write requests to a hard drive. ATA TCQ is not identical in function to the more efficient native command queuing used by SATA drives...
  
   (TCQ) in Parallel ATA and SCSI
  SCSI
  Small Computer System Interface is a set of standards for physically connecting and transferring data between computers and peripheral devices. The SCSI standards define commands, protocols, and electrical and optical interfaces. SCSI is most commonly used for hard disks and tape drives, but it...