Mega Man Star Force (originally known as Ryusei no Rockman) is essentially a continuation of the Battle Network series.
A couple hundred years after the Battle Network series, the Net is now entirely wireless and is known as the “Wave World.” Beings that have “EM” bodies, like Navis, can navigate the Wave World. In some cases humans can go there as well, as long as they fuse with an EM being.
Largely the Star Force games play like the Battle Network games but with a new cast. The series debuted on the Nintendo DS and, among other things, brought the traditional Battle Network battle engine into 3-D.
For more information see the Plot page, or try the Game List.