Answer:
The oldest and major source to Taoism is Tao Te Ching (also spelled Dao De Jing), which translates to The Book on the Way and Virtue. It was written by the legendary Lao Tzu (Lao Zi) somewhere between the 7th and the 4th century BC in China. The old classic still has a lot to teach us.
Tao Te Ching consists of 81 chapters about the Way: its mystery, its cosmology, and what it teaches us about how to conduct our lives the wisest.
On this Tao Themes website, the 81 chapters are sorted according to their themes, which is not the case in the Tao Te Ching, where they appear in an order that seems at least partly to be due to chance - or maybe the writer's impulse, while composing the book. It's also possible that the original book is a collection of proverbs from different sources, done long ago by someone else than their author or authors.
I hope that by sorting the chapters into Tao Themes categories, it will be easier for the reader to examine Lao Tzu's thoughts on different subjects. The Tao Te Ching chapters often return to certain topics, making similar or just slightly altered statements about them. When the Tao Te Ching chapters are sorted according to themes, it's possible to see the patterns of Lao Tzu's thoughts more clearly and to explore them at depth.
Also, this Tao Themes division gives you a chance to focus on certain aspects of Taoism, without having to go through the whole book.