These two desktops each come with a lot of applications, that are not just the desktop. KDE as you suggest, the app names start with a 'k'; kcal, kjots, kfloppy, the list goes on. Gnome is the same. There are many apps with an equivalent in the other desktop.
Some apps will install with the basic desktop, others may need to be installed in addition. For kde, you can go to www.kde.org
and look at the Applications that are available.
Desktops in linux is a matter of choice. You can install as many as you have disk space for. 'buntus' have a default desktop, depending on the first letter. Your choice, which you run. The rest of the system is the same code.