(http://37.media.tumblr.com/a6dd846eb0939ed1ca64f8e2deb7de05/tumblr_mtl9bxhRr01rpsmato1_400.png)(http://24.media.tumblr.com/eb22402c856a4a5976cce7c2ad366a57/tumblr_mtl9bxhRr01rpsmato2_400.png)(http://37.media.tumblr.com/2167078a61e3d9fcc35b9361b2e16b19/tumblr_mtl9bxhRr01rpsmato3_400.png)
(In order from left to right, Neutral Hero, Law Hero, and Chaos Hero. the sauce (http://so-it.rgr.jp/freo/index.php/page/dds_settei).)
The alignment system is a very important part of a lot of the main Shin Megami Tensei games but that'll be explained later.
Summary that I copy and pasted from the Wikipedia page.
(http://upload.wikimedia.org/wikipedia/commons/d/d2/Megamilogo.png)
The first game, Digital Devil Story: Megami Tensei, was based on the novel of the same name by Aya Nishitani. It was released in Japan in 1987 for both the MSX and the Famicom. Alongside its sequel, Megami Tensei II, these are the only two games in the series to have only Megami Tensei in their name. Ky?yaku Megami Tensei (????????), the title of the Super Famicom remake of the first two MegaTen games, translates to "Old Testament Reincarnation of the Goddess". The flagship series then became the Shin Megami Tensei (???????) series, which translates to "Reincarnation of the True Goddess", the Shin in this case being a pun on the shin (??) meaning "new". As a spin-off of Megami Tensei, it eventually became the de facto subtitle of all non-Japanese Megami Tensei releases.
Other games under the Shin Megami Tensei title include Shin Megami Tensei II for the SNES, released in 1994, Shin Megami Tensei if..., also released for the SNES in 1994, Shin Megami Tensei: Nocturne for the PlayStation 2, and Shin Megami Tensei: Strange Journey for the Nintendo DS. An MMORPG, Shin Megami Tensei: Imagine, was released in January 2008, developed by Cave and published internationally by Aeria Games. The game takes place between the first and second Shin Megami Tensei games, and players assume the role of a Devil Buster who ventures into an apocalyptic Tokyo in light of humanity's resurgence, as well as a sudden onslaught of demons. The fourth game in the main series, Shin Megami Tensei IV, was released in Japan on May 23, 2013 for the Nintendo 3DS.
Tl;dr it's basically a first-person RPG-ish game with an amazing story
There's also the Persona series, which I assume you guys are most familiar with Persona 3 or 4 since those are the most popular.
I'm too lazy to write up stuff so i'll just post screenies from each game.
Shin Megami Tensei
(http://lparchive.org/Shin-Megami-Tensei-1/Update%2035/Shin_Megami_Tensei_J_V1.0_T_Eng1.00_AGTP_0405a7.png)
(http://lparchive.org/Shin-Megami-Tensei-1/Update%2035/Shin_Megami_Tensei_J_V1.0_T_Eng1.00_AGTP_003f2e.png)
(http://lparchive.org/Shin-Megami-Tensei-1/Update%2035/Shin_Megami_Tensei_J_V1.0_T_Eng1.00_AGTP_0848b6.png)
(Note, the game is a first person dungeon crawler-esque thingy.)
(http://lparchive.org/Shin-Megami-Tensei-1/Update%2020/Shin_Megami_Tensei_J_V1.0_T_Eng1.00_AGTP_0271d7.png)
Shin Megami Tensei II
(http://lparchive.org/Shin-Megami-Tensei-II/Update%202/3-smt2e_00002.png)
(http://lparchive.org/Shin-Megami-Tensei-II/Update%202/47-smt2e_00045.png)
(http://lparchive.org/Shin-Megami-Tensei-II/Update%203/21-smt2e_00020.png)
(http://media.tumblr.com/3c541b082a001563909aeb9170a46298/tumblr_inline_mws4ansdw81rs46e7.gif)
Shin Megami Tensei: If.... (which is basically an alternate universe sorta prequel to Persona 1 and serves as an alternate ending to the first SMT game?? idk)
(http://lparchive.org/Shin-Megami-Tensei-if/Update%202/4-ddsif_00007.png)
(http://lparchive.org/Shin-Megami-Tensei-if/Update%205/3-ddsif_00025.png)
(http://lparchive.org/Shin-Megami-Tensei-if/Update%205/11-ddsif_00032.png)
(Unfortunately this one hasn't been translated yet... ;____;)
Shin Megami Tensei III: Nocturne
(http://blog.rpgmakerweb.com/wp-content/uploads/2013/06/AudienceSMTN.jpg)
(http://www.projectballad.com/wp-content/uploads/2012/05/nocturne-tower.jpg)
(http://mimg.ugo.com/201110/4/6/4/210464/cuts/nocturne_480_poster.jpg)
(http://24.media.tumblr.com/50042e19841e80b94f1efedf2e7c7e4c/tumblr_mwl8y23W1t1qfokxao1_500.png)
Shin Megami Tensei IV
(http://www.siliconera.com/wordpress/wp-content/uploads/2013/05/smtiv_eng_08_thumb.jpg)
(http://img2.wikia.nocookie.net/__cb20130223014312/nintendo3ds/images/3/33/Shin_Megami_Tensei_IV_screenshot_7.jpg)
ALSO
ABOUT THE ALIGNMENT THINGY
Human characters also possess alignments. There are certain items that can only be equipped to a character of the correct alignment, and the Messian (Law) or Gaean (Chaos) Ashram will charge more money if the Hero is of an opposing alignment. Demons will also refuse to join the Hero if his alignment directly opposes theirs.
The alignment of the Hero is determined by a number of factors. During the course of the game he is asked several philosophical questions (e.g.: "Your mother has been possessed by a demon and pleads with you to end her suffering. Do you kill her?"), the answers to which affects his alignment. Summoning demons will alter the Hero's alignment slightly towards the alignment of the demon summoned. Most importantly, the Hero will be given tasks by both the Law and Chaos factions in the game. Completing one of these tasks will heavily alter the Hero's alignment in favor of the respective faction, while completing both will keep him on the Neutral alignment. The alignment of the Hero directly affects the ending of the game.
This only applies to the main Shin Megami Tensei games I believe.
Persona 1
(https://c2.staticflickr.com/4/3456/3792971968_c803e51633.jpg)
(http://1.bp.blogspot.com/-IwHfXi29RVw/TqSZcic7gSI/AAAAAAAABCQ/L3vLi-5RlIk/s1600/1.jpg)
(http://bulk.destructoid.com/ul/146931-destructoid-review-persona-psp/personapsp_screens_15-620x.jpg)
Persona 2
(http://lparchive.org/Persona-2-Eternal-Punishment/Update%2010/62-Persona2-9-90.jpg)
(http://admintell.napco.com/ee/images/uploads/gamertell/persona_2_innocent_sin_2.jpg)
(http://cdn2.dualshockers.com/wp-content/uploads/2011/11/persona2_screen2-586x332.jpg?57efa4)
(Note: Persona 3 has absolutely nothing to do with Persona 2 and is basically a reboot of the Persona series, hence why it's so different.)
Persona 3
(http://upload.wikimedia.org/wikipedia/en/e/ed/Persona3_battlescreen.jpg)
(http://3.bp.blogspot.com/-nzRcKPjnTWU/UKGoqNfdqPI/AAAAAAAAAH4/_l-2FAW5ujc/s1600/persona_3_35.jpg)
(http://ps2wide.net/img/big/p32.jpg)
Persona 4
(http://media1.gameinformer.com/imagefeed/screenshots/Persona4Golden/p4g_screens_adayinthelife_downtownstroll.jpg)
(http://ugnnultragamingnewsnetwork.files.wordpress.com/2014/03/persona-4-gold-1.jpg)
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(http://37.media.tumblr.com/2a65c35c999235a3b939d9375d75b6ab/tumblr_msys4j7xrl1qcjcsco1_500.png)
Discuss.