I've been wondering, what are all those terminals for.........
what makes aterm different from xterm, more importantly why use an another terminal if you already have a terminal built into x ?

I've never seen a terminal built into X before.

There isn't a lot of difference between the terminals for basic functionality but some allow you to do transparency, open multiple tabs in the same terminal window and I've noticed that gnome-terminal lets you click on URLs and open them in your browser.

xterm is indeed incorporated into X distribution. But it is said to be heavy on CPU and RAM, probably because it has a lot of features (most of those deal with wide compatibility, not eye-candy).

(r)xvt is often prefered because it is much lighter. Gnome-terminal and Kterm are good for tabs. Eterm and aterm are known for transparency (aterm still being light on resources), though transparency can now be done by other (heavier) terminal emulators. Besides, real transparency is arriving with newest X.org.

Things are changing... Just choose the one you like.

Yves.

and a new beast, mrxvt, combines the popular features of rxvt/aterm/eterm (transparency, background image, text shadow, etc) and gnome-terminal/konsole (tabs, freetype font) together, while keep independent from GNOME/KDE environment. it also supports Xorg's translucent window in the newest release - you can specify the opacity degree of a mrxvt window at startup, and dynamically adjust it.