Iaido is an art which you really need to experience for a reasonable amount of time to understand well. No explanation in words can properly transmit the true meaning of Iaido, but after years of practice you will begin to understand.
The scenario of Iaido is peaceful. It’s not a mutually agreed fight like Kendo, it is more like you are attacked as you are walking down the street and you respond to that attack. In Seitei and Mugai-ryu Iaido you are never the aggressor, but merely responding with good timing, to an attack by an unenlightened enemy.
We practice the Zen Nippon Kendo Renmei’s Seitei Kata as well as Mugai-ryu Iai Hyodo. Seitei Iai Kata are taken from various other koryu and adapted slightly for general use after World war 2. There are 12 kata [4 sitting and 8 standing]. For Mugai Ryu, excluding the Naiden, there are 20 kata and 10 kumitachi [paired kata] which we practice. The Naiden are the highest level kata, in Mugai Ryu these are all non-killing kata, and are only taught to the very highest level students.
The (legitimate) Iaido Ryu-ha in existence today are all proven styles. They must be each as valid as each other otherwise they would have died out long ago. Kenjutsu Ryu-ha are handed down by direct transmission over hundreds of years. We learn what our sensei teaches us, our sensei learn't from their sensei, and so on right back to the founder. Styles where the founder was killed in sword duels never prosper as no one wants to follow a style seen as inferior in strength and technique to another. Because of this, the legitimacy of a style and of a dojo is very important.
Iaido uses the katana in a series of kata, which can be thought of as fight scenarios. The kata themselves should not be thought of a set pattern that you would use in a real fight, but are a way of learning waza or techniques, parts of which can be used in a real fight as appropriate. Each kata is a carrier for one or two waza. Generally a kata starts with your opponent deciding to attack you, which you respond to by drawing to cut them, finishing them off and putting your sword away to show that you are not out to kill people, but have merely responded to the attack. The kata themselves are definitely not the essence of Iaido. Doing the kata allows you to do Iaido, and learning how to do the kata well is the preparation you need to do before you can start doing Iaido properly.
Iaido is suitable for all types of people, strong, weak, tall, short, male or female, there are no advantages or disadvantages as long as you have the mental maturity to persevere and the self-discipline to take the time to polish your soul and become a better human being. Men, women and children are all equally able to excel.
Most people mistake Iaido as a lightning fast quick draw, whipping your sword out in the blink of an eye, killing your enemy and returning it to the saya [scabbard] faster than the eye can see, akin to the wild west gunfighters. Although part of Iaido is about killing the enemy before you, the ultimate meaning is about not making enemies in the first place. Iaido’s kanji directly mean matching your action to the situation before you. 居 means to be or exist, 合 means to match or suit, 道 means road or way. The do [道] of all Budo means the road to enlightenment in a Zen sense. In Iaido, because you choose a reaction to suit the situation you find yourself in, that may mean letting the enemy escape, stopping them from drawing or avoiding conflict altogether or instantly cutting them down. Whatever you decide, as a Zen Kenjutsuka. it should be the right and most appropriate way.