When we are coming up with terms to use for various aspects of the system, the first thing we ask ourselves is, "Is this a word or phrase the user sees, or is this a developer issue?"
For things that the user sees, we use words and phrases that describe what the object or action is and avoid technical jargon. So, for instance, we don't use the word "Plasma" in the Plasma user interface except in the About Box. The person logging in probably doesn't really care what that the wonderful thing they see is called, they just like using that thing that, to them, is their "desktop".
For things that the user doesn't see, we try and use technically useful terms. So because the plugin metaclass is call Applet, the .desktop files are plasma-applet-*.desktop by convention. The user never sees this and it helps developers understand the technical path things take. Another example from Plasma is "Containment" versus "Activity": the former is a literal technical term describing the class structure, while the latter is the ultimate functionality it (and a few other related classes) provide to the user.
This leads to slightly different sets of language depending on whether your are dealing with the technical mechanics of things or the user interface. A common term for this is "plumbing versus porcelain" referring to how the user of a sink or toilet sees just the (hopefully) nice exterior and doesn't see any of the internal pipework and supporting mechanisms.
Once you get used to thinking in terms of "mechanism versus purpose" or "developer and user", it all becomes second nature. The alternative of trying to use one language for both developer and user often results in "leaking" jargon out to the user interface (not good) because user language just doesn't map cleanly to the technical implementations in many cases. So instead we have an accurate and concise language for the technology project and a clear and understandable language for the user. Understandably, those who straddle somewhere in the middle (e.g. packagers) can sometimes find this bewildering. ;)
I hope that clears up a few more bits of confusion around the whole "which words to use" issue.