Parallel lines have the same slopes (but different y intercepts). Parallel lines never cross. Parallel lines are the same distance from each other no matter where you are on either line. Think of it like a pair of perfectly straight railroad tracks.
Perpendicular lines cross at one point. The two lines form a 90 degree angle. The two slopes multiply to -1, which is another way of saying that the slopes are negative reciprocals of one another. For example, a line with a slope of 2/3 and another line with a slope of -3/2 has these two values multiply to -1; therefore showing they are perpendicular lines.
