Its difficult to answer this question, for a lot of reasons. Buddha was originally a prince who left the palace and decided to live simply and explore spirituality. Some sects of Buddhism see Buddha as only a great teacher, and some sects see him as something more. I would recommend the book Siddhartha, it is a very good read and can help explain a lot of questions about Buddhism.
Buddha's beginnings and then his development of the belief of a higher power within the person that can attain enlightenment of this greater power, is a positive and peaceful practice compared to many other beliefs of that time.
His history began before the belief of Jesus. Around the 6Th century BC and by the time the Christian Messiah had come on to the scene, Buddhism had grown by the thousands with monks traveling the trade routes through the Roman Empire and the Middle East.
Much of the Christian philosophy based on the teachings of Jesus are Buddhist influenced.
A religion, in most cases, is a practice of a philosophy or spiritual belief with dictates, doctrines, and practices that those who follow that religion must adhere to. So, with that in mind, Buddhism IS a religion, just as Scientology is considered a religion. A religion does not only include the belief in a deity or deities, but a practice of a belief in general.
Buddha is actually a title. It means "enlightened". The person most often referred to as the Buddha is a man called Siddhartha Gautama. There are, however, more Buddhas both preceding and following Gautama, depending on the tradition of Buddhism.
To say Buddhism is not a religion has become fashionable because Buddhism is often viewed as being free of a certain dogmatism that is perceived to exist in the west. However, Buddhism is very diverse and depending on which tradition you're looking at, you will find ritual prayers, deities, an organized clergy, etc..., etc... The more mystical branches do tend to avoid heavy metaphysics, and Zen Buddhists, for example, are well known for espousing a minimalist type of meditation and not saying much about the world at all.
On the other hand, Buddha himself discouraged his followers from inquiring about metaphysics, with such questions as "is the Universe eternal?", "What was there at the beginning?", "Is there a God?", etc..., etc... So in a sense, Buddhism affirms very little about the world and is more interested in develop ping the individual than in expounding a clear worldview. In this sense, Buddhism is less like a religion and more like a philosophy. For this reason also, Buddhism has tended to mix well with other religions. In the Orient, people don't tend to just identify with one faith quite as much and see less of a problem in mixing it up. There are Buddhists that are Taoists, others still practice ancestor worship or Confucianism, etc..., etc...
But don't forget there is such a thing as Buddhist orthodoxy, a Buddhist clergy, etc.., etc...
There seems to be a lot of misunderstanding about this. I would not say Buddhism is more of a philosophy than a religion based upon 25 years of study, practice, and experience with Buddhist groups in Philadelphia, LA, San Francisco, San Jose, and also in Korea and Japan. Buddhism from the beginning (by which I mean the Pali Canon, the earliest record) has many trappings of what we would call "religion" but it is also a culture, a philosophy, a psychology, an ethical system, a way of life, etc...
Anyway, I have written an article about "What is the Dharma" to make it clearer what Buddhism is and what it is not based on the teachings of the Pali Canon:
And I have written another article about how the Buddha spoke of himself in the Pali Canon, and the way he does so might surprise those who have only read secondary or tertiary sources:
In short, the Buddha (according to the Pali Canon or earliest records of his teachings) saw himself as someone who has awakened to reality and who no longer identified himself in terms of form or mental functions the way most beings do. From this we can perhaps say that he had become not just a human exemplar but someone whose life embodied awakening to the unconditioned. Mahayana Buddhism took this type of thinking and even devotion to the Buddha much farther, but really it is just an extension of what is found in the Pali Canon. I do not, however, deal with the Mahayana in that article as I wanted to stick with only those foundational materials.
"Buddha" is a title. It means "the awakened one", or "the enlightened one". It is applied to one who has awakened/become enlightened. Although, unlike many other titles (such as M.D.), there is no governing body who tests and awards THIS title!
There are many Buddhas (many people who have achieved the state of awakening), or so we are taught. But generally when one says, "The Buddha", they are referring to Siddhartha Gautama, the man who became enlightened and created the set of teachings & techniques that are still followed 2500 years later, the teachings called "Buddhism".
Siddhartha Gautama was a spiritual teacher from ancient India who founded Buddhism. In Buddhist traditions, he is regarded as the Supreme Buddha of our age, "Buddha" meaning "the Awakened one" or "the Enlightened one". The most Sacred Teachings in Buddhism are 'The Four Noble Truths'. The Four Noble Truths of Buddhism are 'the' most important teachings within the religion. They are seen multiple times in the sacred texts of Buddhism, known as the Pali Canon - and they are among the truths that Buddha discovered during his journey of enlightenment. 'The Buddha' was simply a human being and never claimed to be more than that. He was not an incarnation of god, a son of god or a prophet of god. All his insight he attributed to his own effort and intelligence. He believed all doctrine should be questioned, even his own. Buddhism is more of a path for the individual, than a faith or belief system. The main central tenants of Buddhism are the Four Noble Truths and the Noble Eightfold Path, for they are the very foundation of Buddhism. When other spiritual teachers and philosophers asked The Buddha to describe his teaching, he would answer: "I teach one thing, and one thing only; suffering and the end of suffering." After his enlightenment, he formulated the Four Noble Truths and the Noble Eightfold Path. *Mindfulness Meditation is a 'very' important aspect of Buddhism. Mindfulness Meditation in Buddhism has two purposes. The first step is to develop mental concentration. The second is to gain 'insight' into the nature of things through 'mindfulness, awareness, vigilance and observation.' When a Buddhist practices meditation, he meditates on Anica (impermanence), Gurkha (suffering), and Anetta (non-self), using the breath as an anchor.
Buddhists see the Four Noble Truths of Buddhism as the way to Nirvana, and all sects of Buddhism hold them sacred. The first Noble Truth is the Nature of Dukkha - all life is suffering. This Truth reflects on the nature of suffering and all of the contexts in which it is seen throughout life. The second of the Four Noble Truths of Buddhism is the Origin of Dukkha which translates to Suffering is caused by Desire. This Truth states that our lives are filled with desire - sexual, desire for objects, for life, even for death. We suffer because we desire many things but we may not always get what we want. This leads to the third Noble Truth - the Cessation of Dukkha. This means to eliminate suffering - eliminate desire. This Truth relies on the fact that suffering can be eliminated, and that people can stop desiring all that they long for within the earthly realm. The final of the Four Noble Truths is The Way Leading to the Cessation of Dukkha. This final Truth translates to the statement to eliminate desire, follow 'the Eightfold Path'. The Buddha taught that hatred, greed, and ignorance were the 'three poisons' that produce the ongoing churn of suffering. Hatred, rage and anger cause us to lash out to the world. Greed, desire, lust and jealousy cause us to crave what we don't have or crave to get rid of what we do already have. Ignorance is simply our delusion about the world-we don't perceive that it is impermanent, always in flux, and has no self-nature. The Buddha's remedy or antidote for the three poisons was to follow 'The Noble Eightfold Path': right view, right intention, right speech, right action, right livelihood, right effort- (which includes the five precepts), right mindfulness -(mindfulness meditation), and right concentration. The Four Noble Truths of Buddhism may be hard to understand at first, but once one undertakes the process of studying Buddhism and learning about this religion, they will gain insight into how these Truths fit into life.
SOURCES;
Buddhanet really is quite excellent-for it contains a self-study course on Buddhism based on the Historical Buddha, His Teachings, and Buddhist History & Culture: http://www.buddhanet.net/e-learning/index.htm
"Eight Mindful Steps to Happiness" By: Henepola Gunaratana - thoroughly explains The Four Noble Truths and how to walk The Noble Eightfold Path.