Penzai is an ancient Chinese practice of growing small plants in pots. Penzai literally translates to "tray scenery", but loosely means potted landscape. The practice moved through Asia and eventually into Japan. Bonsai trees, a Japanese pronunciation of Penzai, became popular during the Heian period in 794 to 1185.
They are not meant to serve a purpose beyond art and contemplation. It is a study in balance, restraint and harmony, as they serve no medicinal or culinary purposes. They don't contribute to larger landscapes either. For this reason, Bonsai is a very narrow practice, focusing on one or two plants.
This is so important in Japan that one of the oldest known living specimens is considered a National Treasure. It is thought to be at least 500 years old. It is on display in the Tokyo Imperial Palace.
They can be made with nearly any material. As long as it is perennial and has woody growth, it can be made into one. It also should branch and remain small in containment with root and crown pruning.
There are some favorites in this form though. These have characteristics that make them visually appealing on the compact scope of these miniature landscapes. These characteristics usually include small leaves or needles. Some favorites include ficus, because it is easy to grow, many of the evergreen trees, because of their unique wood textures and small needles.
Source materials can be found in many different places. Source material is what is used to begin the process. Bonsai are very difficult to start from seed. Most growers don't want to start from seed either because the ultimate goal is a mature and weathered appearance. It would take many, many years to achieve this, so most growers use source material that is already mature or partially grown.
There are many intricate techniques in forming a tree.
Bonsai trees are a meditative hobby with deep cultural ties. These beautiful sculptures can bring joy for generations. Any level horticulturalist can take on one of these projects for years or happiness.