Here's why:

* Transverse waves: The particles of the medium vibrate perpendicular (at a right angle) to the direction the wave travels. Imagine a rope tied to a wall. If you shake the rope up and down, the wave travels horizontally along the rope, but the rope itself moves vertically.

* Longitudinal waves: The particles of the medium vibrate parallel to the direction the wave travels. Think of a slinky. If you push and pull the slinky, the compressions and rarefactions (areas of closer and farther apart particles) travel along the slinky, and the slinky itself moves back and forth in the same direction.

Examples of transverse waves:

* Light waves

* Electromagnetic waves

* Waves on a string

* Water waves (to some extent, as they also have a longitudinal component)

Let me know if you'd like more information on waves!