Gap represents an area where there has been no trade.Different types of gap.
*Area Gap -Area Gap occurs inside the trading range
*Break out GAP-Breakout Gap occurs only when price breakouts from the range.
*Runaway GAP-Post breakout,the runaway scenario take place,runaway gap appears inside the area.
*Exhaustion GAP-The gap appears after a trending zone seems to be a runaway gap,but if it gets filled on a same day then it can be considered to be exhaustion gap.