It's essentially a keyboard lacking function keys, a numerical pad, and a navigation cluster. Usually has between 61 and 68 keys, and contains an extra "function" key that acts beyond the typical Fn key to help you control layers. Every 60% doesn't have layers, but most do, and those layers are generally programmable. These days pretty much all of them use QMK or VIA firmware.

The Deskthority article for some reason mentions the Minila which is actually a 65% keyboard (which is a 60% + arrow keys).