Gaps are also known as 'Rising Windows' or Falling Windows' in Japanese Candlestick charting.
An opening 'gap up' is when stock/index starts a new trading sessions at a higher price than the previous days close. An opening 'gap down' is the reverse of this - when the stock/index opens the day lower than the preivous close. They are especially significant when coupled with an increase in volume.
Gaps come in three different kinds - Breakaway Gaps, Exhaustion Gaps, or Runaway Gaps (sometimes also known as Continuation Gaps).
This usually occurs after the conclusion of a major price pattern and precedes the start of the next price movement. It signals potential change in the trend and is more significant when accompanied by increased volume.
This takes place at the end of an upward or downward trend and signals the conclusion of that particular movement. For an Exhaustion gap to be a signal the price should reverse shortly after the gap and it should be filled by new price movement almost immediately.
 Runaway Gap
This is sometimes also known as a 'Continuation Gap' and will appear mid way through a strong upward or downward trend.
All types of gaps in a market price can act as areas of support or resistance to future market movement.
 See also