A SCSI card is a card that will control the interface between SCSI versions of hard drives, CD-ROM drives, CD-ROM burners, removable drives, external devices such as scanners, and any other SCSI components. Most fit in a PCI slot and there is a wide range of types. The three main types of connectors on these cards are 25-pin for SCSI-1, 50-pin for Narrow SCSI, and 68-pin for Wide SCSI (and Ultra-Wide SCSI, Ultra2-SCSI, Ultra160 SCSI, and Ultra 320 SCSI - all of which use a 68 pin connector).
SCSI controllers provide fast access to very fast SCSI hard drives. They can be much faster than the IDE controllers that are already integrated your computer's motherboard. SCSI controllers have their own advanced processing chips, which allows them to rely less on the CPU for handling instructions than IDE controllers do.
For the common user, SCSI controllers are overkill, but for high end servers and/or the performance freaks of the world, SCSI is the way to go. SCSI controllers are also much more expensive than the free IDE controller already included on your motherboard. There is also a large premium in price for the SCSI hard drives themselves. Unless you have deep pockets, there isn't much of a point in going with a SCSI controller.
Many people buy SCSI controllers just for use with their CD-ROM burners and CD-ROM drives (these drives must be SCSI drives of course).
SCSI cards also have the ability to have up 15 devices or more per card, while a single IDE controller is limited to only 4 devices (some motherboards now come with more than one IDE controller though). SCSI cards allow these drives to be in a chain along the cable. Each drive on the cable has to have a separate SCSI ID (this can be set by jumpers on the drive). The last drive on the end of the cable (or the cable itself) has to "terminate" the chain (you turn termination on by setting a termination jumper on the drive - or use a cable that has a terminator at the end of it).
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