Soft tennis is a kind of tennis which was developed in Japan based on Regular tennis.
The major difference compaired to regular tennis is that much softer rubber balls are used in soft tennis.
This causes to have developed unique features in soft tennis, such as lighter and more flexible rackets with lower string compression to fit for the soft balls and unique techniques and game maneuvering to make the best use of the softness of the ball. You can hit the ball hard with a good and comfortable feeling and will find it easy to control the ball. In a doubles match, which is more popular in soft tennis, you can see breathtaking, varied rallies and destructive net plays. You will also be attracted by exciting and unique game maneuvering and impressive combination/cooperative plays shown by paired players. I assure you that you can enjoy these attractive features of soft tennis not only as a player but also as a spectator of soft tennis.
The history of soft tennis is as long as over 100 years and the current number of its players is as many as 5 million in Japan. Internationally, soft tennis has been popular in Korea and Taiwan, and gradually spread to the world. Since 1975, the World Championships are held every 4 years.
(reference; World Soft Tennis Information-- Here is more detail from WSTI.)

I've played tennis since I was in junior high school. While I play regular tennis as well as other sports such as soccer, basketball, and baseball, soft tennis is on top of my list. The appeal of soft tennis are
1. Voice; You can express your emotions as much as you want in the match. It's like a roaring party.
2. Touch; When you hit a ball, firstly you catch a ball with your racket, just a few seconds you hold the ball which gets squashed, the ball is shooted getting its original shape with good clean sound, and you can savor the exciting touch. It's irresistible.
3. Collaboration; If you are a baseline player, you maneuver the games. If you are a net player, you smash the balls. You can also share the joy and enjoyment with your partner and with your team. It's combination with your partner and collaboration with your team that leads you to victory.
Finally, for me, "No soft tennis no life".