Traditional Buddhist teaching has a fair amount to say on the subject of Maitreya (called Maitteya in Pali).
First and foremost is the belief that Maitreya is already alive- simply not on this earth. Traditional Buddhst beliefs hold that before being born in the world, a Buddha passes one last life as a deva, among the gods of the Tushita heaven. Traditional sources describe Maitreya as dwelling in Tushita heaven even now, awaiting his rebirth in the world of form.
When Maitreya will be reborn is a matter of cosmology. To be called a 'Buddha', a being must rediscover from scratch that path that all previous Buddhas have taught- the Four Noble Truths and the Eightfold Path. In order for this path to be rediscovered, it must first be lost; Buddhist tradition holds that Maitreya will be born only after the Dharma has declined to a point where it is forgotten by human beings, and no trace of the previous Buddha and his teachings can be found. This period of decline and contraction is described in the Lotus Sutra and other sources, and is often compared to the decline that Hindu belief terms the Kali Yuga.
While the memory of the Buddha Shakyamuni remains in the world, some believers feel that enough time has passed that the true path to enlightenment has been lost. This belief commonly recurs when Buddhism is threatened by an external force, such as a war or the persecution of the Sangha. Buddhists who subscribe to the belief that the true teachings have been lost believe that the best that one can do is to make merit and perform virtuous acts, in the hope of being reborn in the future with the opportunity to hear the Dharma from Maitreya. The prayer 'come Maitreya come' is sometimes found scrawled on boulders or carved into ritual objects in Tibet and other Buddhist areas.
Contrary to whizkid's words above, traditional Buddhist beliefs hold that a Buddha must by necessity be male. This requirement is specified in a number of sources, including descriptions of the marks of a Cakkravardin ("Wheel turning Monarch"), as well as in jataka tales and their commentaries, and a few other predictions and prophecies. Mahayana Buddhism introduces wrinkles into this idea, presenting occasional images of enlightened females (such as the devi in the Vimalakirti Sutra), but Theravada Buddhism presents similar images in the Therigatha ('Verses of the Elder Nuns'). While maintaining that awakening transcends all bounds of form and gender, Mahayana Buddhsim does not ever go so far as to claim that a Buddha can be female with any clarity. Keep in mind that a Buddha is distinct from other awakened beings; both Mahayana and Theravada Buddhism acknowledge the possibility of female awakened beings, but both claim that male birth is a prerequisite for Buddha-hood.
Maitreya is a figure that is found in both Theravada and Mahayna scriptures. In the Pali Canon, he is mentioned primarily in connection with his future birth in suttas in the Digha Nikaya and elsewhere. In the Mahayana canon, he gets a bit more coverage, putting in a personal appearance in the Lotus Sutra and a few other scriptures. However, it is likely that Maitreya is best known for being present in Chinese restaurants all over the world. Folk lore holds that the Chinese monk Po Tai (transliteration may be incorrect; his name means 'hemp sack') was an incarnation of Maitreya. Po Tai, sometimes called the 'Chinese Santa Claus', was a jolly, chubby, lovable monk known for giving toys to village children from his bottomless sack (from which his name was derived). At his death, Po Tai revealed himself as an aspect of the Bodhisattva Maitreya. His beloved figure- palatable to both Buddhist and Confucian values- became a fixture in Chinese culture, giving rise to the 'fat Buddha' that one sees in restaurants and other settings.