Buddhism begins with the Four Noble Truths, that I paraphrase as:
Buddhism addresses unnecessary suffering, rather than metaphysical or teleological issues such as the nature of things, or why things are as they are.
Another translation of Buddha's word "Dukkha", rather than "suffering", is misalignment, largely between ego and one's true or original nature. Other translations of Dukkha are dissatisfaction, unsatisfactory experience, anguish, misery, or similar limited words. Much of this suffering is fueled by delusions stemming from habitual discursive thinking driven by correlated but partly subconscious emotions such as anger, hatred, greed, attachment, clinging, ignorance, sloth, and anxious worry, as well as jealousy, envy, avarice and pride.
The Four Noble Truths are a diagnosis, and the 8-Fold Path prescribes what to do; The Heart Sutra and Metta Sutra offer more detail, about a page each. The Heart Sutra addresses form and emptiness, and mind and consciousness. The Metta Sutra addresses kindness and compassion. You can find Sutras easily on the web.
Perhaps the key principle of Buddhism is the Middle Path, balancing between opposites such as ascetic austerities and self-indulgence. "There is addiction to indulgence of sense-pleasures, which is low, coarse, the way of ordinary people, unworthy, and unprofitable; and there is addiction to self-mortification, which is painful, unworthy, and unprofitable ... the Middle Path gives vision, gives knowledge, and leads to calm, to insight, to enlightenment and to Nirvana ... The Middle Path is just this Noble EIghtfold Path ... "
Other principles of Buddhism include no abiding self, transience of experience, impermanence, emptiness, and causation. Impermanence and emptiness imply that we have no permanent self. Instead everything changes, including our egocentric notions of who we are, and what self is. Causation, or karma, implies that things happens due to prior causes and conditions.
Buddhist practice engenders awakening and liberation. Enlightenment is central to Buddhism. Buddhist lore includes awakening to unsurpassed, complete, perfect enlightenment, e.g., Buddha's enlightenment. However, most Zen students experience awakenings or increasing openness over the years, as well as a subtly increasing sense of transparency within ourselves and our practice, and a sense of being increasingly permeated by the very sweet energy of nature, nirvana, and emptiness.
There is no single source in Buddhism. Instead each country and tradition has separate organizations, and they are quite diverse. Some are faith-based, like western religions. Among many notable sources the Dalai Lama (Tibet) takes kindness as the core of Buddhism and happiness as the core of life. But, while kindness and happiness can be core, and practice is key, there is still a great deal of Dukkha in life. Also notable, Thich Nhat Hahn (French/Vietnamese) emphasises practice; see my review of Thich Nhat Hahn's "The Heart of Buddha's Teachings".
Buddhism is relatively free of rules and beliefs. Instead Buddha urged students to practice relying on their own experience. Stephan Batchelor, in his book "Buddhism without Beliefs" characterizes Buddhist Practice as what Buddhists do with respect to Buddhist principles.
Thich Naht Hahn's small book "Be Still and Know" includes "Peace is all around us, in the world and in nature, in our bodies and our spirits. Once we learn to touch this peace, we will be healed and transformed. It is not a matter of faith; it is a matter of practice." Peace is one ideal among many, e.g., love, compassion, kindness, truth, and equanimity, and Thich Naht Hahn's statement applies equally to each ideal. Realizing and manifesting ideals can be approached through practice.
From Dogen's book "Beyond Thinking", edited by Kazuaki Tanahashi, "spiritual practice is not some kind of striving to produce enlightenment, but an expression of the enlightenment already inherent in all things." Just after his enlightenment and attainment of Buddha nature, Buddha purportedly said something like "it is wonderful that all things, and all beings, are enlightenment".