Nearly all music games are based on the principle of notes floating up or down the screen; as the notes reach targets, the player presses a button corresponding to the target and is judged based on his timing - each note is then graded on the level of accuracy achieved, with the highest levels adding to a combo count that continues as long as successive note grades are at or above the combo level.
A life bar measures whether or not your performance is sufficent to pass the song and continue play. At the end of the song, you recieve a score and overall grade.
In StepMania's default game type (similar to Dance Dance Revolution), four types of arrows, left, down, up, and right, are used (plus Numpads 2, 4, 6, and 8 in case of Double play). Players use a "dance pad" with a panel for each arrow that is stepped on.
Each song has a difficulty level, and most official songs contain multiple difficulties for different levels of play.