The Dharma is the Buddha's doctrine. According to the Buddha's own words, its essence could be resumed in this way:
All the phenomena are arising from a cause. The Buddha, the great virtuous one, Taught about the causes of phenomena And the end of these causes: Don't commit any fault, Flawlessly follow the virtue, Completely master your mind; This is the Buddha's doctrine! The practitioner who preserves a totally pure behavior, Perfectly controlling his body, his speech and his mind, Will surely free himself from all sufferings.
The Buddha gave a great number of teachings (traditionally it is said that he gave 84.000 different teachings), which correspond to the different tendencies, wishes and capacities of beings. A way to present these teachings is by dividing then into three 'Vehicles': Small, Great and Adamantine.