After a market forms a pattern, it eventually starts a new trend higher or lower. A breakout occurs when prices move through either the supporting or resisting border of a chart pattern. News or market events cause the perceptions of the bulls and bears to change and one or the other becomes more aggressive in their activities.
If it is the bulls, then the market goes up. If it is the bears, then the market goes down.
Breakout occur with sudden shifts in perception that causes increased activitiy from an increased number of market participants. Volume usually rises and prices usually move in wider intraday ranges.