The Battle Network series was introduced to the Game Boy Advance in Japan in 2001 under the title Battle Network Rockman EXE. This is actually the star’s name as well—he’s a software A.I. known as Rockman.EXE or MegaMan.EXE in English.
When Capcom, the game’s creator, brought the game over to English, they reordered the title a little into Mega Man Battle Network. It tells the story of Lan, a fifth-grader whose best friend is his Net Navi, MegaMan.EXE (or just MegaMan for short). Throughout the game he battles computer viruses and the Net Navi of other operators. He also finds Battle Chips to give MegaMan.EXE new attacks.
The Battle Network games could probably be best described as a cross between a typical video game RPG and a battle trading card game. Battle Chips can even be traded with friends, or gambled on a Net Battle.
Although the games began on the Game Boy Advance, the series eventually branched to other systems as well. After a while, the “Mega Man NT Warrior” brand emerged and became quite well-known as it was used for the cartoon shows, the English translation of the manga, the trading cards, and just about everything relating to the series outside of the games themselves.
For more information see the Plot page, or try the Game List.