It is any type of herb from which certain phytocannabinoids are extracted. Certain compounds that are contained with the plant itself that can be gathered in a high enough concentration to have an effect on the partaker. (Essential oils are a good example of this) When you combine the extract with other compounds (oils, herbs, vitamins, minerals, etc.) that is what is called a tincture.
In regards to CBD, the extract is usually combined with Hemp oil (from which the CBD is also extracted) and can be combined with other herbacous plant extracts for compounding desired effects. For example: our AWARENESS Tincture