The difference between a worldly being and a Buddha - according to the teachings, a Buddha is perfect in the Paramis.
A Buddha is free of all greed, hatred, and ignorance. A Buddha has more wisdom then any being in the world. A Buddha is able to fully enlighten himself, without a teacher to show him the way. A Bodhisattva is a being working towards becoming a Buddha. According to the Theravadin teachings they can't be enlightened. Because if one is enlightened at any stage, stream-enterer, once-returner, non-returner, or Arahant, then one has actually become enlightened through the teaching of another Buddha. A Bodhisattva, in Theravadin Buddhism, is a being working towards bringing the teachings back to the world without a teacher to show them the way to fully enlighten themselves. According to Theravadin Buddhism, that Bodhisattva long ago met a Buddha in another life. When he did, he had the potential to become fully enlightened in that life, he was already a renunciant, an ascetic, possessed all the psychic powers, and the Buddha had to recognize that this being is making the vow to become a Buddha, and then that living Buddha has to recognize whether that aspiration is going to fruit. So there is quite a high requirement in Theravadin Buddhism to become a Bodhisattva, and they can't be enlightened yet.
An Arahant is a person who has become enlightened through the teachings of the Buddha, the Dhamma, and is also free of all greed, hatred, and ignorance. According to Theravadin Buddhism they have ended the cycle of rebirth for themselves and are fully purified.