Kamae is to be differentiated from the word tachi (立ち?), used in Japanese martial arts to mean stance. While tachi (pronounced dachi when used in a compound) refers to the position of the body from the waist down, kamae refers to the posture of the entire body, as well as encompassing one's mental posture (i.e., one's attitude). These connected mental and physical aspects of readiness may be referred to individually as kokoro-gamae (心構え?) and mi-gamae (身構え?), respectively.
Although it is a generic term, context may mean there's a default specific posture which is being implicitly referred to. e.g. many modern styles use kamae by itself as shorthand usually for the style's basic stance for sparring or self-defense.
As a further note, there are also related verbs, and adding te to the end of kamae makes the command for "get ready/in position" (構えて kamaete?). Thus, a karate instructor ordering the students to assume a front stance might shout, "Zenkutsu dachi, kamaete!"
Kamae is a basic stance, also defined as natural. In it, the body's three centers of gravity are aligned on a vertical axis of gravity. Those three centers begin with the head, then spinal column, and lower abdomen. This allows for a balanced stance, regardless of positioning of one's feet. It also allows one to move freely into any desired direction.
There are five basic kamae in Kendo: jōdan, chūdan, gedan, hassō and waki. Of these, chūdan-no-kamae (middle stance) is the most basic posture. It provides a balance between attacking and defensive techniques.
There is only one form of kamae in karate, where the Martial artist places his right fist guarding the plexus and the left fist at shoulder height. This to quickly adapt to each situation where the Martial artist may counter-attack with his right hand and guard with his left (or vice versa depending on the stance).