Buddhism rejects the concept of soul (soul is viewed as an extension of the identity - oneself). Most people find it difficult to comprehend as people are used to think in terms of tangible, or countable items/concepts. This is why most people have trouble in understanding what karma is.
Karma is sometimes referred to as 'Kamma wega' which is a form of thinking that is much easier to understand. Simply put, your thoughts/actions translates to karma. If you take a Newton's Pendulum, provide energy to one end, the rest of the pendulums will transfer energy to the next. But there is no physical transformation apart from the energy. In a similar manner, what you do, will create karma which would determine your next life (and events in this as well). Your thoughts/actions translate into karma, which has the capability of affecting you in this life and many others to come. This is how the world works, be it you belief or not. This is a universal law to all beings in all 31 planes of existence. This is an endless cycle. Which means that you cannot ever complete the process. That is why Buddhism urges to break the cycle rather than trying to complete it.
Budhdha is not a god. He is a man. A human being who has achieved the ultimate mental state anyone can achieve, without the help of any master. That is why he is known Samma Sambudhdha.