Largest Religions: Buddhism
Buddhism is both a religion and a philosophy that is based on many different traditions and teachings. Buddhists do not worship a god as such but rather they look to find enlightenment internally. There are actually several different sects of Buddhism that have some radically different teachings, so much so that it can be hard to determine just who is a Buddhist.
Buddhism is a religion that is based on the teachings of the Buddha who is believed to have lived in India in the fifth century BC. The details of his life are spelled out in a biography that explains the path that he took to enlightenment. That being said most scholars believe that this should be taken with a grain of salt. The only thing that they are willing to say for sure is that he lived and that he founded a monastic order. It is the teachings of this monastic order that form the basis for Buddhism.
Given that he was from India it is not a surprise that Buddhism shares much in common with Hinduism. In fact many scholars believe that Buddhism can be considered to be a branch of Hinduism. This makes it hard to determine just how many Buddhists there actually are. There are several different sects that consider themselves to be Buddhist but many of them can also be considered to be Hindus. The best estimates put the number of Buddhists at around half a billion but that may not be a particular accurate guess.
One of the reasons that it is so hard to differentiate who is a Buddhist and who is a Hindu is that many of the Buddhist sects have very little in common. In fact the only aspect that connects all of them is what is known as Dependent Rising. This is the belief that phenomena occur together in a mutually interdependent cause and effect. Basically there is cyclic effect with everything that occurs giving rise to the next thing that occurs. This is the basis for the Buddhist belief in reincarnation. Beyond this the teachings of the different sects can vary dramatically.
One other aspect of Buddhism that most, but by no means all, sects believe in are the four noble truths. Even these are not agreed on completely as they are worded and interpreted differently by the different sects. The most common wording however is that the first noble truth is suffering. The second noble truth is the arising of suffering. The third noble truth is the end of suffering. And the fourth noble truth is the way leading to the end of suffering. In some Buddhist sects the noble truths are advanced teachings for those who are well on their way to enlightenment. In other sects they are a preliminary lesson that needs to be taught before the quest for enlightenment can start.