Ninja Gaiden is a series of fast-paced action games by Tecmo. The series was originally named Ninja Ryukenden, but it changed when the series came to North America. Gaiden literally means "side-story," usually a word reserved for spinoff games. In the PAL region, the series is known as Shadow Warriors. Later installments in the series have been developed by Team Ninja.
The series has had a variety of installments (many just titled "Ninja Gaiden"), spanning many consoles. The series gained popularity in the original Nintendo Entertainment System trilogy for its fast-paced, gritty action, detailed graphics, and cutscenes (one of the first games to feature this innovation). The series has appeared (in various forms) on the arcade, NES, Game Boy, SNES, Master System, Game Gear, Xbox, Xbox 360, PlayStation 3, Nintendo DS, and Wii Virtual Console. A beta version has also been uncovered for the Mega Drive (Sega Genesis in North America). Various other ports on PC or other systems have been created, but they haven't reached the popularity of the other versions.
Ninja Gaiden Shadow on the Game Boy isn't technically part of the Ninja Gaiden series, as it is a licensed edit of the NES game Shadow of the Ninja, from Natsume.
The Ninja Gaiden franchise has had many different gameplay changes over the years that the series has been on the market.
Ninja Gaiden (Arcade)
Ninja Gaiden's first entry as an arcade brawler has been overshadowed by the popularity of the ambitious action platformer released later in the same year on the NES. The player takes control of an unnamed ninja as he travels to various locations in North America, fighting a cult.
Other than sharing a name, this title is not considered canon to the official series and storyline.
NES, SNES, and Game Boy
The installments on the NES, SNES, and Game Boy are all of the same gameplay. The NES games introduced many ideas that have now become standard in the series. The trilogy introduced the "Ninja Arts" (also known as "Ninpo"), detailed story cutscenes, incredibly challenging difficulty level, and the various power-ups of the series. The games centered around Ryu Hayabuse fighting an ancient evil that a cult headed by a character known as Jaquio is intent on resurrecting to his benefit.
The three NES games were remade for the SNES game Ninja Gaiden Trilogy. Not many things were improved besides graphical detail and soundtrack (which made the game receive some critical reception), but the game is still somewhat of a rare collector's item.
Ninja Gaiden Shadow on the Game Boy isn't technically part of the Ninja Gaiden series, as it is a licensed edit of the NES game Shadow of the Ninja, from Natsume. However, it shares much of its gameplay with the Ninja Gaiden series.
The series saw a revival in 2004 with Ninja Gaiden on the Xbox. It differed from its NES counterparts, making the gameplay into a 3rd-person action game instead of the action platformer that it used to be. The series has since seen several other titles on modern platforms, including Ninja Gaiden Black, Ninja Gaiden Sigma, Ninja Gaiden: Dragon Sword, and Ninja Gaiden II.