Originally, In the Groove (ITG) was a 4-panel dance game created by Roxor Games based on the earlier (and much more popular) Dance Dance Revolution (DDR) game by Konami. Like DDR, arrows rise to the top of the screen, to the beat of the music, until they reach hit targets for left, up, down, and right arrows. The player has to hit the right buttons when it crosses the hit target to score well and prevent failing the song.
ITG ran on a modified StepMania 3.95 engine (an open source clone of DDR) and introduced features such as mines and hands and support for “mod charts”. ITG was sold as a PS2 game, an upgrade kit for existing DDR cabinets, and as an ITG dedicated cabinet (dedicab). Shortly after ITG gained popularity, it got into legal trouble with Konami and it went away in an official capacity.
This video does a great job of summarizing the game.
The term "ITG" originally was used to refer to In The Groove, the game series developed by Roxor. But over time, ITG colloquially changed to mean ''4-panel StepMania played on a dance pad with ITG judge and life values''. So if you're using a 4-panel dance pad, have OutFox, use life and judge 4 values (which you would be by default if you use a theme like Simply Love), then people would say you're playing ITG. This term is often debated and on its face is inaccurate, since you're not playing the original In The Groove game -- but it's just a manner of speech you'll hear online a lot so it's worth explaining.
ITG uses judge 4 and life 4 in Stepmania. See Judgements and Timing Windows for more.
Stepchart Creation
Stepcharting Beginner Tips by ?
Doubles Charting Guide by Brother Mojo
Writing Lowers Charts by valgrind
Charting Crossover Patterns by sorae
Stamina Patterning Guide by domitg
Writing Stamina Prototype by Rems
Advanced Stepcharting by Brother Mojo
Marketing Your Simfiles by @@
Marketing Your Simfiles by Zaia
Null Sync & Pack.ini Guide by C Jota
Stepchart Editors
Basic Guide for ArrowVortex by Fietsemaker
SM5 (Scroll) Gimmicks Guide by C Jota
Stepchart Asset Creation
GIMP Graphics - Basic Tutorial and Walkthrough by Chief Skittles
Animated Simfile Banners by Telperion
ITG Syncing Guide Episode 1 [Video] & Episode 2 [Video] by JNero
Audio Normalization Guide by Jasmine
Using Audacity to Cut Songs [Video] by dimo
StepMania Graphics Tutorial [Video] by dimo
Dance Pad Reviews
Gameplay
Intro to ITG Tech Notations by Vincent
4-Panel Form Guide by iamchris4life
Online Lobbies in ITGMania by cmmf
ITG Tournament Prep 101 [Video] & 102 [Video] by dimo
Ghost Stepping: 3 Tips to Immediately Improve Dance Game Scores [Video] by dimo
Custom Dance Pad Builds
Mystic DDR/ITG Pad Build Guide (Arcade Sensors) by bluemoonmystic
Sereni's Travel Pad Build Guide by Sereni
Teejusb's Travel Pad Build Guide by teejusb
Azirixx's Travel Pad Build Guide by Azirixx
FSR Travel Pad Build Guide [Video] by Goose
Portable Travel Pad w/ FSR Sensors by tomnobar
Dance Pad Modifications
StepmaniaX FSR Tape Mod Tutorial [Video] by Miklowcic
SMX Gen 1-3 Load Cell Sensitivity [Video] by StepRevolution
Arvin's Pad Modding Tutorial by Arvin
Building a bar for my L-Tek pad by Roujo
Utilities
ITG Difficulty Predictor by som1sezhi
Pack.ini Generator by Jasmine
Bermuda Triangle by bkirz & Ash Astral