To me, the zheng instrument looks like a string instrument (e.g. violin, cello, viola, etc.), but played in front of you like a piano. It's limited to the number of strings compared to the number of keys on a piano, but it's more complex than a violin or other string instrument because it has more strings . Here is summary of the history of the zheng:
Bakan, Michael B., World Music: Traditions and Transformations. 2nd edition. New York: McGraw Hill, 2012. ISBN: 978-0-07-352664-5.
If you know anything about Chinese history, you would know that it has one of the longest documented histories of all nations. As you can see in the image above, this is the zheng history post 1912. This is only a microscopic piece of Chinese history. The zheng has evolved over the ages primarily due to politics. The more freedom China became over the generations, the more developed the zheng has grown.
As I mentioned before, the zheng is set in front of you like a piano, but played like a guitar or cello. As for the sound, for the most part it is mostly known to be played with very soothing and peaceful sounds. It's sound is also known to communicate sadness, enlightenment, and romance. When it's played in a film to represent action and suspense (e.g. Crouching Tiger, Hidden Dragon film), even though you might feel scared or excited, you feel like you still have a sense of control and security from the smooth sounds it makes. You could say that it is the oxymoron of sounds when you play it using scary sounds because it's mostly known to be used to make peaceful ones. The zheng has had many lifetimes to be perfected and just when you think it's sounds perfect the way it is, someone and/or something comes along and finds a way to make it even better.
Here is a link to the zheng instrument being played in the 4 time Academy Award winning Crouching Tiger, Hidden Dragon film: