If you are connecting hard drives directly to your server through the motherboard or add-in cards, without leaving the case itself, that's referred to as Direct Attached Storage (DAS). The other alternative is to use an external interface, usually Fibre Channel or Ethernet, and connect a Storage Area Network (SAN) or Network Attached Storage (NAS) to hold the database disks. SAN and NAS hardware is typically much more expensive than DAS, and easier to manage in complicated ways. Beyond that, comparing the two is somewhat controversial.
There are external drive arrays available that attach over SAS or eSATA cables, so they appear as direct attached storage even though they are technically external to the server ...
