st is a simple terminal implementation for X.
What is wrong with xterm ?
xterm is bloated and unmaintainable. Here’s an excerpt from the README:
Abandon All Hope, Ye Who Enter Here
This is undoubtedly the most ugly program in the distribution. It was one of the first "serious" programs ported, and still has a lot of historical baggage. Ideally, there would be a general tty widget and then vt102 and tek4014
subwidgets so that they could be used in other programs. We are trying to clean things up as we go, but there is still a lot of work to do. More Info »
Needless to say things have not changed, it’s still ugly. It has over 65K lines of code and emulates obscure and obsolete terminals you will never need. The popular alternative, rxvt has only 32K lines of code. This is just too much for something as simple as a terminal emulator; it’s yet another example of code complexity.