There are hundreds of differences in herbal medicine. So, you should figure out how you are going to use it before learning about it.
One of the oldest and most popular paths is the Traditional Chinese Medicine path. This is a vast range of materials and philosophies. It works and is proven in many scientific research paper (mostly in Chinese). Chinese medicine is used in nearly half of the world as the primary form of care.
It's also highly regulated and difficult to learn. You need to learn the Chinese names, often the Chinese script, philosophy of meridians and energy paths. What turned me off is that nearly half of all the herbs I would learn about I would never have the chance to see growing live. I would be forced to import nearly everything I worked with.
Now, if you are a hand's off person, this is good for you. But, I wanted to grow my own herb and know where they came from.
Ayurveda also has a rich and old herbal system. It too has a strong scientific basis and it works very well.
This too is also regulated and you need to learn another path of philosophy. What makes it better than Chinese medicine is many of their herbs are also foods. The most common ingredients are cooking spices.
But, I didn't want to learn another language or philosophy just to use herbs. Plus, more herbs than I was comfortable with would need to be imported.
Western Herbalism encompasses everything from Europe to America to the Middle East. It has a long history, but one that is troubled with Crusades, Inquisitions, and modern medicine bias. In Europe and the Middle East, herbalism is learned with surgery as a primary tool for physicians. In the US, herbalism is considered witchcraft.
Personally, I feel US doctors are being taught to monopolize the healing practices for the profit of the AMA and pharma industry.
In all herbalism, you have the professional practitioners, the medical practitioners, and the home practitioners. In addition, you have those who use any herb they like and those that use only local herbs.
The Wise Woman of old, or the medicine man was a local home practitioner. That is what I wanted. I took my classes, combined it with the education I had in science, and the hands on learning from a good teacher. Now, I use about 20 herbs regularly and I can either grow them myself or get them local. They help me and those I'm close to to be healthy and happy.