Arahat: free from craving
Theravada understanding: Bodhisattas are aspirants to attain Buddhahood. Did not speak much about what Bodhisattas realize or practice, apart from the cultivation of paramitas. The only Bodhisatta mentioned is Mettaya (sanskrit. Maitreya) Bodhisatta, who currently resides in Tusita heaven waiting for birth to become the next Buddha.
Mahayana understanding: Bodhisattvas are aspirants to attain Buddhahood. Apart from the cultivation of paramitas which are very important to a Bodhisattva, the Bodhisattva can be categorized into either pre-bhumi or entered-bhumi Bodhisattvas. There are ten stages, bhumis, to attain Buddhahood. The pre-bhumi Bodhisattva engages in practices such as the cultivation of paramitas and contemplates on the nature of reality to obtain insight, however he has not attained realization. The entered-bhumi Bodhisattva obtains the realization of the twofold emptiness: the emptiness of self, plus the emptiness of objects. Other than Maitreya Bodhisattva, many Bodhisattvas are popular in Mahayana texts: Avalokitesvara Bodhisattva represents great compassion, Manjusri Bodhisattva represents great wisdom, Ksitigarbha Bodhisattva represents great vows, Samantabhadra Bodhisattva represents joyful giving and great activity.
Theravada understanding: Buddhas are fully awakened beings, who are not only liberated but are also omniscient, confers perfection of paramitas, mastery of psychic powers, and a whole lot of qualities. Does not speak about what is the difference of realization between Buddha and Arhat, apart from brief and vague mentions in suttas like 'arhats have understood things' and 'Buddhas have understood things all the way to the end', etc.
Mahayana understanding: Buddhas are fully awakened beings, who are not only liberated but are also omniscient, confers perfection of paramitas, mastery of psychic powers, and a whole lot of qualities. Has perfected all skillful means for the sake of delivering mass sentient beings to the shore of liberation. Bodhisattvas and Buddhas have deeper understanding of emptiness, and they do not rest in the cessation of feelings and perceptions but can manifest freely to liberate sentient beings. They abide in the Nirvana of non-abidance, neither clinging to samsara nor cessation.