When it first appeared in the Street Fighter series, the Dragon Punch, a jumping uppercut attack, was both the ultimate anti-air move and counter attack, due to its invincibility on the way up. This meant that any attack that collided with the Dragon Punch would be defeated, though the character would be completely vulnerable on the way down, so that launching the move carelessly was still inadvisable.
As the the Street Fighter series progressed, the Dragon Punch lost its invincibility, and much of its ability to beat other moves, to the extent that in many cases, a standing kick makes for a better anti-air attack.
There are several variations of the normal Shoryuken, such as the Shoryurepa, a series of flaming Dragon Punches done in succession, or the Shin Shoryuken, a much more powerful variant that begins with a strike to the abdomen and connects into the Dragon Punch.
"Dragon Punch" is also used as an umbrella term for any jumping uppecut attack that resembles a dragon punch, or functions similarly. As virtually every fighting game has some sort of dragon punch derivative move, the term is quite common. The typical input command for the Shoryuken, the socalled Z-motion, consists of moving the joystick forward from neutral position, then down and then rolling it from down to down-forward (creating a Z-like motion) and hitting an appropriate attack button. This input motion has become a staple in many 2D fighting games, and because of its origin, is often simply referred to as a Dragon Punch motion, even if the resulting move doesn't even have Shoryuken-like properties.
In the original arcade versions, the attack was referred to as Sheng Long, due to a translation error; at the end of each victory, Ryu would say, "You must defeat Sheng Long to stand a chance." However, most players assumed that Sheng Long was a hidden character instead. For the SNES console version the error was corrected, but a mention of a "Master Sheng Long" in the game's manual further fueled the rumour. Popular gaming magazine EGM played off this in their 1992 April Fool's prank, where they revealed the (fake) method of unlocking Sheng Long. The hoax has since gone on to partly influence the creation of the Akuma and Gouken characters.