What is an hda on Linux?

It is the 6th partition of you primary IDE hard drive.

hd stands for hard disk ( Please note that in case of SCSI the names would be sd)

Disks are numbered as hda, hdb, hdc and so on

partitions are numbered as 1, 2, 3 and so on

So hda1 is the first partition on first hard disk and hdb2 is the second partition on second hard disk.