Buddhism is the path of the Buddha Dharma: a way of understanding reality, the causes of suffering or unsatisfactoriness, and the means to end it. It can be approached as a religion, a philosophy, and a practice of mindful living.
As a religion, Buddhism is unusual because it does not center on a personal creator God. As a philosophy, it investigates reality, knowledge, ethics, and mind. As a practice, it trains awareness, compassion, and wisdom through meditation and daily conduct.
At its heart are the Four Noble Truths: life involves suffering, suffering arises from attachment, attachment can end, and the Eightfold Path shows how. Buddhism therefore is not only something to believe, but something to realize and live.
It is also expressed through three major branches: Theravāda, Mahāyāna, and Vajrayāna. Each interprets the Buddha’s teaching in a distinct way, while sharing the same basic aim: liberation from attachment and suffering.
Theravāda Buddhism is the closest to the actual teachings of the historical Buddha. Mahāyāna Buddhism, and subsequently Vajrayāna Buddhism, are significantly different, to the point where they can be considered a different religion. They invented their own scriptures and doctrines, and did not uphold all of the earlier teachings.