The combined factors of getting a late start this month, the lack of notable releases, and the general length of games in the SMT series has led me to combine August and September for the SMT theme. The Megami Tensei (Rebirth of the Goddess) series is developed and mostly published by ATLUS games. The "Shin" was later added for the third game. The original game was released on the Famicom in 1987 and the sequel in 1990 (both published by Namco). These games established the basics that have carried through the mainline games in the series: first person dungeon crawling, demon catching, negotiation, and fusing, and a cyberpunk theme. There have been 11 games in the mainline SMT series, though not all have been released outside of Japan. There have also been a large number of spin-off games that share at least some of the themes, including the Digital Devil Saga duology, the Devil Summoner games, and the Devil Survivor games. The most well known spin-off is the Persona series, which grew so popular that it has produced its own side games.
Generally, the games are all stand alone properties, though the Persona games have some connections (especially Persona 3 and 4, as the characters have interacted in several of the spin-off games). The two Devil Summoner games are direct sequels as well, and the Digital Devil Saga games are a self contained story to themselves. There are often multiple endings available in the SMT games, depending on choices the player makes during the game. Many times these choices are more complicated than just good or evil, and involve law, chaos, or varying philosophies. Generally speaking, the SMT games are also known for their difficulty and specific planning is often needed for bosses due to the fusion system, which does not reward mindless grinding. Directed grinding is usually very important, though.
The combat system can change some between the games, but they all use similar terminology and basic strategies. While they have created their own methodology for spell names (e.g. bufu=ice), they are consistent from game to game with them, so you generally only have to learn them once. In combat, there is a particular system used in which weaknesses and turn order are very important. It is handled differently from game to game, but generally it is critical to learn what the weaknesses of your opponent are (elemental or otherwise), and just as important to protect your own weaknesses.
Some of my favorite RPGs of all time are included in this series, and I hope everyone gets a chance to explore this franchise at some point. Enjoy!