This concise article describes K2 (version 2 of the K language), and, at the level it describes, little has changed, so it is still a good introduction to the language 12 years later.

However, beyond the basics, a lot has changed since (current commercial version is K4, current dev is K6 AFAIK): Dictionaries have become better integrated in the language, the database layer was rolled into the language, the integrated (bare bones and milspec-looking but ultra effective) electric GUI was dropped, http and web (as both client and server) were added to the core, 64-bit integers, nanosecond timestamps, GUIDs were added as core types, and probably a few more things I forgot.

If you find this article interesting, you may want to experiment with JohnEarnest's ok [0], which provides graphical playground (implemented in JS and runs everywhere) and also read Q for mortals[1] - Q is a syntactic-sugar version of K4, though still the same language underneath.

[0] https://github.com/JohnEarnest/ok

[1] http://code.kx.com/q4m3/