A SCSI host adapter is a device that is used for connecting one or more SCSI devices to a computer bus. A SCSI host adapter is usually known as a SCSI controller, which is not strictly accurate, as any component that understands the SCSI protocol could be termed a controller. In this perception, all SCSI devices have a SCSI controller integrated into them, whereas host adapters are responsible for transferring the data between the input/output bus of the computer and the SCSI bus.
SCSI host adapters function as a significant adapter for FireWire ports.
SCSI host adapters are also known as SCSI adapters.
Techopedia explains SCSI Host Adapter
PCI cards are the most advanced SCSI host adapters, which are either 32 bits or 64 bits. The older adapters were centered on the transitional 32-bit VESA and EISA buses or on the 16-bit ISA bus. The SCSI host adapter can be natively integrated into a PC's motherboard, but this usually makes the motherboard very costly. It is best to use a standalone SCSI host adapter as it can be effortlessly transferred or replaced.
SCSI host adapter types are as follows:
Basic, comparatively low-performance cards: This type of card offers a cost-effective solution to connect SCSI devices that do not need a substantial amount of bandwidth, for example, SCSI scanners and CD-ROM drives. These types were quite common until 2000; however, most of them became obsolete afterward. The innovations in IDE technology, especially in the case of CD writers and CD-ROM drives, added to their downfall. Also, the introduction of the Universal Serial Bus (USB) made SCSI interfaces avoidable for the majority of scanners.
High-end cards: These cards are built for optimum functionality, generally for incredibly high-speed hard disk drives and particularly for scenarios that demand multiple high-speed drives (especially servers). These cards are comparatively expensive.
With the arrival of Serial Advanced Technology Attachment (SATA), the use of high-end SCSI host adapters is expected to decline further.