Classic Series
Mega Man "classic" reffers to the original series of Mega Man games. The first game, Mega Man, was introduced in December of 1987 on the Famicom and Nintendo Entertainment System (NES). Most games in the series are referred to as "Mega Man" (or "Rockman") followed by a number (2, 3, etc.). However, the ninth game in the classic series (not counting the "side-stories") is called Rockman & Forte, or Mega Man and Bass.
The basic story of the game revolves around robot engineer Dr. Thomas X. Light, robotic "children," Rock (a lab-assistant) and Roll (a housekeeper), and their struggle against Light's former colleague Dr. Albert W. Wily. As the series began in 1987, Capcom set it in the early-21st century. The first two games Mega Man and Mega Man 2 (as well as some of the side-stories) take place in the ambiguous years of 200X (sometime between 2000 and 2009). However, Mega Man 3 and beyond set a more ambiguous date of 20XX (presumably 2010 or later, although it could be anytime between 2000 and 2099).
X Series
The Mega Man X series made its debut in December of 1993 on the Super Famicom and Super Nintendo Entertainment System (SNES). The series is set in the 22nd century, almost one-hundred years after the events of the Mega Man classic series. The story revolves around Mega Man X, the final creation of Dr. Thomas X. Light, who is discovered by world-renowned archaeologist Dr. Cain and used to create an advanced race of robots known as "Reploids" (called so because they are literally reproductions based on the design of X). However, not long after their creation, some of the Reploids begin to go "maverick" and attack humans. Because of this, Dr. Cain is asked by the council to put a stop to this, and as such, forms a special unit to stop these "Mavericks" known as the "Maverick Hunters." However, the leader of the Hunters and Dr. Cain's initial creation, Sigma, goes maverick himself, taking many of the Hunters with him. Thus, the beginning of the First Maverick War begins to take shape.
Zero Series
The Mega Man Zero series made its debut in 2002 on the Gameboy Advance and takes place in the mid-24th century, one-hundred years after Zero had himself sealed in order to "cleanse" his body. In this time-period, both the "Reploid Wars" and "Elf Wars" have ended, but a new threat has arisen and threatens to destroy both human and Reploid kind. A young scientist named Ciel, a member of the "Resistance" (later called the Guardians), accidentally comes across the ruins of the lab in which Zero was sealed and awakens him. Thus, Zero becomes a member of the Resistance and takes up his sword once again to fight for the forces of justice.
ZX Series
The Mega Man ZX series made its debut in 2006 on the Nintendo DS and is set about a century after the events of the Mega Man Zero series. In the 25th century, the difference between humans and Reploids has become nonexistent, and a new combination of "man and machine," called "humanoids" has emerged. Although the series of events known as the "Second Maverick Wars" has long since ended, a new series of "maverick" uprisings begins to occur, which proves to be a dangerous threat since all beings on earth are now part machine in nature and thus susceptible to the outbreak. Vent and Aile, orphans from a maverick outbreak that occurred ten years ago, get drawn into the new conflict, along with Giro, the head of their delivery team.
Legends Series
The Mega Man Legends series made its debut in 1997 on the Sony PlayStation. The series takes place thousands of years after the events of the Mega Man ZX series, taking place sometime in the 9th millennium (sometime between 8001 and 9000 A.D.) in a world largely covered by water. The series is also called "Rockman DASH," which stands for "Digouters Adventure Stories in the Halcyon Days" (which in Egyptian Mythology was supposed to take place in the 9th Millennium A.D.).
The story is centered around Mega Man Volnutt, Roll Caskett, and their "grandfather" Professor Barrel Caskett as they search the world, "digging" through ancient ruins, in search of Quantum Refractors (the major source of power for their civilization). Along the way, they skirmish frequently with the Bonnes, a group of marauding pirates and their servbots.
Battle Network Series
Sometime in the not-so-distant future, the internet has become integrated with every piece of electronic equipment in existence (from TVs to ovens), and people carry around hand-held devices known as a Personal Exploration Terminal (PET). These devices act as phones, pagers, instant messengers, news beams, and hacking devices all rolled into one. Additionally they are able to be customized with a "NetNavi" or a human-like personality that can be anything the owner desires.
A fifth-grader named Lan has a NetNavi named MegaMan.EXE, a virus-fighting blue-suited cyber personality who is also Lan's best friend. Although MegaMan.EXE has no body (unless the PET device itself counts), he can wander around in cyberspace and explore computer systems as if they were alternate worlds. Together Lan and MegaMan.EXE (also known as just MegaMan) cooperate to fight off hackers and viruses.
The relative peace of Lan's hometown is shattered when electronic devices begin mysteriously going berserk. Talk of a takeover by the "World Three" ("WWW") abounds. MegaMan and Lan decide to tackle the problem from two sides-the real world and cyberspace. It's going to take everything they have to counter the mastermind behind all of this.