hda is the first hard disk (first controller, master) and hdb is the second (first controller slave I think). Nr. 1 is the first (primary) partition on it. You can have up to four primary partitions, that is for 1 to 4. If you have a extended partition on your harddisk and have logical partitions in it, they start to number with 5, for example your second logical partition in your extended partiton on your first harddisk would be called hda6.
The first primary partition on your first harddisk will be called hda1
EDIT: Please note that hda1 therefore is usually the partition Windows will install to (and I guess your Windows install resides there, too)