Tom,
My 2-cents.
Hi 
Programs with tk (or more usually gtk) at the end or at the beginning are for a one type of DE for Gnu&Linux. Sometimes a G is used instead. The other main type of DE usually has K or Qt at the front of it's programs.
"tk" stands for Tool Kit, so truncating GTK (Gnome Tool Kit) is not entirely proper.
Often programs have a "back-end" or "command-line" tool that does most of the heavy lifting and then different "front-ends" or "Gui"s are put on for each of the 2 main types.
Typically we talk about families of distros but even a single distro might have 2 or 3 versions with each one having a different type of DE. If you choose the 'wrong one' then you can choose whether to install the other DE or get a different version of the distro that does have the 'right one'. Tim at Kracked Press has somethings he likes in each of the main DEs so he installs both. It makes his system a bit more bloated but means he can use choose more apps.
DE = Desktop Environment. The main 2 are Gnome and KDE. Most of the rest (Xfce, Unity, Enlightenment and probably hundreds more) tend to be able to use front-ends written for one or the other.
I have switched to Xfce, since IMHO KDE 4 is still a basket case even though it is release 10 (4.10) on my Slackware 14.0 distro! From what I am hearing on other forums, I am not alone in switching. The K people had a very stable KDE in 3.5 and they were only up to release 5 (3.5). I was sorry to see that stability go for an entire rewrite in KDE 4.
Ok, so it's not quite that simple. 2 extra wrinkles;
1. Gtk or tk are pretty rarely used but are for the Xfce DE (well really a WM (=window manager (note the lower-case w)) but that is nearly a DE) and Xfce apps work well in Gnome.
Not so. From my experience, most of the apps without a "K" prefix are written with GTK, so they can run on the most DEs. I ran GTK apps on KDE.
Gnome is a bit heftier (a bit more "fully functionally" if you know what i mean) so it's fairly normal to find a G (stands for Gnome) instead of the rarer Gtk but then that's a pain because the app might need a 3rd front-end instead of just having 2 to reach everyone.
2. Going back to seeing the K at the beginning of apps written for KDE makes sense but why the Qt? Well, until recently Qt was less streamlined and was a lot of the weight in KDE. Now it is a lot faster and lighter it seems that Gnome or distros using Gnome have pulled it in but just not quite enough of it for Tim's requirements.
3. Since Gnome often can run apps built for the 3 main DEs shouldn't that make it the DE of choice!? Oddly not. It's been forked in at least 2 or 3 different directions and in Ubuntu it's been replaced by Unity (which can also run a lot of the Gnome, Xfce or KDE apps but is extremely unpopular amongst purists)
KDE is and has been, built on the QT libraries, so the QT is redundant. K* can assume QT. Most if not all of the other DEs are built on the GTK libraries. In my experience, there are many more applications built on GTK than QT. Apps built on GTK will run on KDE, however, I am not sure apps built for KDE will run on all GTK DEs. I know for a fact that KDE apps will run well on Xfce, I am doing so. In fact, I was amazed at how well they do run. The QT library was proprietary at one time (Trolltech). I don't know if the current version is. GTK is open source (GNU) licensed.
I hope that helps!! I hope i got it about right too otherwise i'm going to get deluged with unwanted flaming or something! Something i like about Gnu&Linux is the passion and that we go all sorts of different ways but somehow manage to grow and learn from each other or make use of each others achievements and even build on them (if individuals are gifted enough)
Regards from
Tom 
Yours in enlightenment.
Girvin Herr