Any app the is made from blackberry or RIM is not considered a 3rd party app. etc. facebook for blackberry, twitter for blackberry, blackberry messenger and so on. Anything from any other vendor is considered a 3rd party app.
RIM classifies their apps as Add-on. If you got to the applications list on your BB, you'll see there is a list for 3rd party, add on and core.
Yes you'll see those lists, but it's not exactly accurate to say "RIM classifies their apps as Add-on."
The Add-on section are (mostly) native RIM apps that you have the option to install or not during OS installation and that you are permitted to delete.
A few non-native apps get included in Add-on such as Sudoku by Magmic, Docs To Go by DataViz, etc.
@OP: the 2 best places to find 3rd Party Apps are BlackBerry App World and the CB Superstore.
(The CB Superstore is "powered" by Mobihand.)
Both have their apps to go on your device. I have both personally. Both have flaws and advantages. They are organized in different ways.
Try them both out!
Posted from my CrackBerry at wapforums.crackberry.com
Appworld is easier to navigate in my opinion but is over-priced and slow to update the software when a newer version is released. Crackberry usually updated their software much faster and has items on sale more often. I usually use Crackberry but made the mistake at first of using Appworld(aka Crapworld) when I got my first BB.