Perpendicular lines are those that form a right angle at the point at which they intersect. Parallel lines, though in the same plane, never intersect.
Another fact about perpendicular lines is that their slopes are negative reciprocals of one another. The two slopes multiplied together always produce an answer of -1. The formula for slope is rise divided by run. If a line goes through point (x1y1) and point (x2y2), the formula for its slope is set up as y2 - y1 / x2 - x1. If the slope of one of the two perpendicular lines is 1/2, the slope of the other is -2.