The game series is based off the 7th gen console war with the addition of the fictional Sega Neptune, in which the main character is also named after. Each are a personifcation of said console, PS3, 360, Wii, and Neptune, each having their own world(landmass) they govern over. People(players) have faith with each goddess which gives them their powers and it's a struggle of superiority of who is the true goddess. The main villain is Arfoire(pronounced R-4 in reference to the pirating device) who wishes to destroy Gameindustri(their world).
The games humor is very gaming geek level stuff, everything from old school games to newer console stuff is all present. Each world is made of stuff that you would expect out of said console so in Lowee(Nintendo) you can expect alot of Mario References and things of that nature. The game is also very meta as well and it constantly breaks the 4th wall...very much so intentionally.
As the series progresses their are alot of inside jokes for players who started since the beginning and it seems to be that way in each installment. The first game was very generic and lackluster in pretty much everyway but the Skits. So...if you treat the game as a Visual Novel with a tacked on RPG...the game is quite enjoyable. Later games in the series are actually better as an RPG, Victory pretty much got it right and the remakes uses the Victory battle mechanics.
If your interested in the series I suggest playing Rebirth 1 and go from there.