So Siddartha left his father's palace behind and joined a group of ascetics, monks who practiced self-discipline and self-denial. Long story short, he didn't find what he was seeking with them either, so he departed, and sat down under a tree and meditated. It was there that he found enlightenment. The actual story is a lot more dramatic than this, but I'm summarizing.

Basically, it is the self that leads to suffering, which is the cause of the continual cycle of reincarnation. So Buddhism basically embraces selflessness. Siddartha recognized the following:

1) That everything is impermanent
2) That every action has consequences (hence, karma)
3) Change is possible

The eightfold path of Buddhism is based on these basic facts. The reason that he is called the Buddha (Enlightened One) is because once he achieved this enlightenment, he could have released himself from the world and become one with the cosmos, giving up his self entirely. (No heaven in Buddhism, only nothingness.) Instead, he remained to help others find the path.