The Buddha was living with a group of ascetics who practiced the denial of all comforts to the body. They ate and slept extremely little, never bathed, and lived outside. When the Buddha awakened, and realized that this was not the way to enlightenment, he jumped into the river to bathe and began drinking milk again. His companions, disappointed that he had abandoned his spiritual practices, tried to pay him no attention but when the Buddha approached them, they were so moved by the transformation he had obviously undergone that they were compelled to place their hands in gassho and bow. The Buddha replied, "Wonder of Wonders! All sentient beings have this same [enlightened] nature." The ascetics were not bowing to their old companion Gautama, or to the Buddha and his enlightenment, but rather to the enlightenment in all things.
Gassho ("ga-sho"; Japanese for "place the two palms together"; Korean: hapchang; Sanskrit: anjali) is the Buddhist term for holding your hands palm-to-palm, fingers extended, at about chest height. There seems to be a bit of a discrepency about what exactly each term means in each language: gassho is at face-level, a hand's width away from the face, with the arms either nearly parallel to or at around 45° angles to the floor; hapchang is chest-high, with elbows roughly at 45° angles to the floor; anjali is also at chest level. Gassho is commonly followed by or included in a bow, but hapchang can also be used as an expression of respect or to say "no thanks," and anjali is used in India as a gesture of greeting, farewell, respect and thanks.
Gassho is common to all sects of Buddhism, both Mahayana and Therevada, and there are many forms of gassho. The two most common are formal gassho (hands at the face, arms almost parallel to the floor), used in rituals, services and meals; and mu-shin (no mind) gassho (hands at chest or face level, not pressed together so tightly, arms at 45° angles to the floor), used in greetings and as a show of respect. Other forms include renge (lotus) gassho (hands apart by an inch; used by monks in rituals) and kongo (diamond) gassho (fingers straight and interwoven; also used by monks in rituals). Jodoshinshu Buddhism has twelve forms of gassho.
Gassho is also a form of architecture with which I am not familiar.