Next: Gyakuzuki Dachi

This ring is under construction so please don't nail me for broken links. It's likely to take about a month to finish, I'm about 1/8 of the way through. Thanks for your patience.

Junzuki Dachi is named after the Junzuki technique. This is a forward punch off the front arm. As you might expect Junzuki is usually performed in Junzuki Dachi, though a variety of other techniques are also performed from this stance.
Junzuki is one of the Zenkutsu Dachi family of stances. These are forward-weighted meaning that more weight is placed on the front leg than the back. The stance can be formed by standing with feet fogether (Heisoku Dachi) and then sliding one foot forward to about 1 and a half to 2 shoulder widths. The rear foot turns outward by about 30°, and the front knee is bent, typically around 30-45°. The upper body varies from front facing to half-side facing depending on the specific technique being employed.
Since the most obvious thing to do in Junzuki Dachi is a Junzuki, I will explain that here too. Extend the arm on the same side as the front leg out in front and make a fist. The fist should be directly in front of your solar plexus region. The rear hand is also formed into a fist and the fist is placed at the side of the hip above the straight leg with knuckles facing upwards. The shoulders should be forward facing, and back up straight.

Here's the problem though: I have seen a lot of different ways of forming Junzuki Dachi. I actually do it differently to the method stated above. The reason I gave the above definition is because this came from the most reputable source I've found so far. However other variations usually differ in the positioning of the front foot.
In my style (based in Wado ryu) the stance is formed from a ready stance with feet shoulder width and then the front foot extended exactly as above. However because of the initial width of the feet, the stance similarly has a shoulder width horizontally as well as the 1.5-2 shoulder width length vertically. Also in our style the hips and shoulders are locked in a forward facing position. This limits the reach of the stance slightly but together with the wider foot position makes the stance very stable and strong.
In a Shotokan club I trained at for a while, the stance was similar to the one I just described (ie slightly wider than the 1st) but their stance was a little shorter, usually at 1 and a half shoulder width long. They had more flexibility in the position of the hips and shoulders too using forward, half side and reverse half side facing torso positions.

As I already mentioned, Junzuki Dachi is used for Junzuki. (I keep wanting to say 'Junzuki punch' but that's a bit like 'RAM memory' or 'PIN number'). Typically this is taught to Karateka after they have started to master Sonobazuki (punching on the spot), and involves a stepping motion with turning, blocking and gradually the introcudtion of more punch techniques and some kicks. I am not going to go into much depth on this subject here because there is a node dedicated to the theory and basics of Tsukiwaza (straight punching). This is covered in detail therein.

Junzuki Dachi is closely related to Gyakuzuki Dachi, Junzuki Tsukkomi Dachi and Gyakuzuki Tsukkomi Dachi. These 4 stances together with the Kokutsu Dachi (back stances) and in particular Nekoashi ('cat' stances) make up the staple of stances used in the first few Karate Kata, especially the 5 Pinan Kata.