In Mahayana Buddhism there are many Buddhas and they are seen as cosmic beings that may choose to appear in human form to teach others. Although they believe that shakyamuni Buddha did follow the Bodhisattva path to achieve Buddhahood he had in fact been enlightened for eons and his appearance is one of several. He did not in fact die but rather he chose to appear to so that he may help us learn his teachings as they will not become too reliant on him and it will encourage them to seek their own path. This belief has been formulated into three separate bodies which make up the Trikaya doctrine. These bodies are: Nirmanakaya, Sambhogakaya, and dharmakaya.
Nirmanakaya is described as the transformation body; this is the earthly manifestation of the Buddha that he uses so that he can teach us more easily. Buddhas can create any form that they want to, and even have the ability to be several different forms at once if they so wish. Some see these as only being a sort of manifestation of the Buddha but others see them as genuine “flesh and blood” beings. This has obvious parallels to be drawn with Jesus Christ as he is the bodily incarnation of God in Christian beliefs, the idea that God had to make himself man to help teach us is closely related to the fact that Buddhas must make themselves men to help teach us as well.
Sambhogakaya is sort of a bodily form that Buddhas and other advanced bodhisattvas take; they can become accessible to bodhisattvas and those who are in the heavenly realms. They can appear in visions during meditation practices and offer teaching to those who are less enlightened than themselves. They have the ability to create “pure lands” where through the power of their own enlightened minds they can create a world which is helpful for those who are trying to attain nibbana. In these lands it is easier to understand and practice the Dharma. Other Buddhas create “Buddha fields” within non pure lands