In Buddhist thought, a bodhisattva (Sanskrit) or bodhisatta (Pali) Thai: โพธิสัตว์ phothisat) is a being who is dedicated to attaining Enlightenment. Bodhisattva literally means "enlightenment ('bodhi') being ('sattva')" in Sanskrit; it also refers to the Buddha himself in his previous lives.
In the Mahayana tradition, Bodhisattvas take vows to work for the complete enlightenment of all sentient beings. A Bodhisattva strives to become fully enlightened as a Buddha so as to have the best abilities to help other beings, and takes the vow to not enter into (passive) Nirvana before all other sentient beings have achieved complete Buddhahood. This method is not really taught in Theravada philosophy, where indeed the majority of Enlightened Beings are Arahants who achieved Nirvana, not Buddhas.
The meaning of Bodhisatta in the Pali Canon and the Theravada tradition does not imply that a Bodhisatta made the vow not to enter Nirvana until everybody else is enlightened: this is a Mahayana innovation. Therefore, the 'Bodhisatta' and the 'Bodhisattva' are quite different in nature.
The Bodhisattvas are honored in many famous artworks, including one of the highest sculptures of the Bodhisattva at the Chinese Puning Temple, built in 1755.