The triangle can be a continuation or a reversal pattern. Although, more often it is a continuation pattern. There are three types of triangles: symmetric, ascending, and descending. For trading purposes they are all the same, the just look different.
A triangle forms when the price action narrows over several price swings. If trendlines are drawn along the highs and lows of the price action, the trendlines converge towards each other. This creates the appearance of the triangle.