I found an application that requires XP Pro. I was told that "he
main difference between XP Home and XP Pro is, you can't join Windows
domain with XP Home." What is "Windows domain" and why would I want
to join it?
Why do you care (since obviously you are NOT logging into a domain)?
Your concern is that the unidentified program demands it be installed on
Windows XP Professional. So now your choice is to toss the program or
upgrade from XP Home to XP Pro. Depends on how important is that
application to you.
If you want to use the unidentified application, and because it demands
XP Pro, then you'll have to upgrade to XP Pro. Really doesn't matter
what Pro has over Home since the choice to upgrade is decided by whether
or not you want to use this unidentified application.