HBAs are host bus adapters. They are a type of special purpose network card specifically designed for talking to storage systems. Typically via Fiber Channel, SCSI or eSATA.
Host Bus Adapters are special purpose "network" devices for talking to storage systems. They use storage protocols (SCSI, Fiber Channel, etc.) to communicate with a SAN or set of disks.
They can be expansion boards installed in a server, or an on board option, much like 'conventional' network cards.
Because of the specific needs of storage they tend to be lower latency than you would get from a conventional ethernet network card.
