ATAPI stands for Advanced Technology Attachment Packet Interface. It is also called ATA Packet Interface.
ATAPI is a protocol applied to parallel ATA or Serial ATA. It enhances the capability of ATA and allows an increased variety of devices to be connected to a computer as comparison to ATA.
ATA was initially designed for work only with hard disks and devices that could emulate them. ATAPI was introduced by a group called Small Form Factor committee (SFF) . It made ATA able to use for a variety of other devices that require functions beyond those necessary for hard disks.
For example: Any removable media device needs a "media eject" command, and a way for the host to determine if the media is present, and these were not provided in the ATA protocol. ATAPI protocol allows the ATA interface to carry SCSI commands and responses. It makes all ATAPI devices are actually "speaking SCSI" other than at the electrical interface that's why ATAPI is also known as "speaking ATA".
The common ATAPI devices are: