Blitz is the German word for lightning. A "blitz game" refers to a game where the thinking time is limited. Typically, this means a 19x19 game with less than 15 minutes each side, no overtime; or else a game with very short main time and overtime of not more than 10 seconds per move. Normal rules apply apart from the shorter time limits.
Some players like to adopt unusual playing styles in blitz games with the aim of confusing the opponent in order to win on time. There may also be other reasons for adopting unusual styles. See /discussion.
Sometimes beginners are encouraged to play blitz in the spirit of the proverb "Lose Your First 50 Games As Quickly As Possible".