Daoism is an organized religion native to China. It's known as both Taoism and Daoism. However, the pinyin style of translating Chinese is the official standard now so all future publications spell it as Daoism.
The root word of Daoism is Dao, usually translated as the Way. The word Dao isn't unique to Daoism. Confucianism, and other Chinese traditions also used the word Dao. In a sense, it is sort of like Truth or Dhamma or Way of Life. Each group argued over what the Dao was. But Daoism particularly adds a particular meaning to the word.
Daoism involves ideas of naturalness, simplicity, and going with the flow. Like Confucianism, it's extremely influential in East Asian aesthetics, medicine, health, cooking, and so on. You can see Daoist influences in Tai Chi Quan (slow, internal martial art), Qi Gong (body-mind energy work), acupuncture, Chinese medicine, and so on.
However, there is also a social, religious, and political part of Daoism involving complex rituals, priests, other worldly soul traveling, image meditations, and spells/magic.
Personally, I am a religious elitist in the sense that I'm not as interested in the "popular religion" or "folk religion" aspects that are often talked about. I learned about Indian and Chinese religions from a philosophy department so my focus is more on understanding how the founders and most advanced members understood the world rather than what are local worship rituals or how a religion influenced politics. So, I'm going to focus mainly on the "philosophy".
Almost everyone identifies Daoism with the Dao De Jing by the author Laozi. Actually, there is a pretty good argument that we can trace Daoism even farther back to a Nei-Yeh text (Inner Trainings), but that has been a recent development. Westerners, starting with the Catholic Jesuits, understood Daoism as philosophy and religion and thought the religious side was a corrupted side. Today, scholars think there's a lot more interaction between the two.
At this point, I want to mention a problem when talking about Daoism or religions in general. Daoism spans over two thousand years. When Daoism or Confucianism or even Christianity began, no one called it by those names. They were just following the teachings and practices of a spiritual teacher. As time goes on, a lot of things are assimilated, changed, or removed from the religion. So, for example, the idea of Yin and Yang that's often associated with Chinese thought wasn't particularly Daoist at first. Likewise, the entire pursuit for immortality or the magic spells were already around but got appropriated by Daoists. Today, Daoists including the monks aren't trying to reach immortality (as far as I know) just like most Christians don't believe in indulgences anymore.
I think it's partially justified because the Dao De Jing and Zhuangzi are very philosophical. There's no manual for beliefs or practices or rituals or priesthood anywhere in these texts. Arguably, Daoism and Confucianism began as ways of life and Daoism only later became what we think as a organized religion. Not to mention any book on Daoism will begin with Laozi and the phiolosophy before it goes into the religion.
