A RAID controller — the thing that directs data in and out of storage drives — can be based on either software or hardware. In the former case, if you have a few drives set up in a RAID array inside your home PC, it's likely handled on a software level with your PC's processor (CPU) handling the work. In the latter case, say with an external Network Attached Storage (NAS) enclosure, there will be a dedicated controller card to deal with input and output (I/O) operations of the RAID array.