What's the difference between internal and external hard drives?
Are internal hard drives the ones that come with the computer when you just bought the computer?
Can any type or brand of internal hard drives go and work with any type of computer?
Internal and external desktop hard drives are the same. It's just that external desktop hard drives comes in a shell that allows you to use it outside of the computer case.
All desktop hard drives are mostly the same. Unless you have a recent computer, try buying one that is "IDE" or "ATA". Don't get a "SATA" or "Serial ATA" unless you know for sure your computer can use it.
There is also a notebook hard drive which are smaller than desktop hard drives.
You can tell by their sizes; desktop hard drives are 3.5" wide and notebook hard drives are 2.5" wide.
Notebook hard drives tend to cost more than desktop hard drives so that's another way to know. Don't get a notebook hard drive for your desktop as there's no point in doing so.