Here's why:

* Transverse Waves: The particles in a transverse wave move perpendicular (at right angles) to the direction the wave travels. Think of a rope tied to a wall. When you shake the rope up and down, the wave travels along the rope horizontally, but the individual pieces of rope move vertically. The crests (high points) and troughs (low points) of the wave correspond to where the rope is moving upward and downward, respectively.

* Longitudinal Waves: In a longitudinal wave, the particles move parallel to the direction the wave travels. A good example is sound waves, where the particles in the air compress and expand (move back and forth) in the same direction the sound is traveling.

Let me know if you'd like more details or examples!