It is not important to know what the letters stand for, just that you know what style your current drive is. Try checking your sales invoice or manual. If you can pop the lid of your computer and read the model and manufacturer's name off the drive, someone can tell you what it is. Virtually all drives purchased in the last 3 years are either SCSI or IDE, with IDE being the most popular. Almost all hard drive controllers will support 2 hard drives.
SCSI controllers can support many more.
If you are planning on keeping your existing controller and hard drive, you must make sure that the new drive is the same type. To be frank, if your old drive is an MFM or RLL, you should seriously consider replacing it with an IDE drive. And the access speeds would be dramatically faster. You don't want to try to get two different type hard drives with two different type controllers to work on the same machine. Accomplishing this takes a lot of work.