Here's why:

* Transverse waves cause the medium to move *perpendicular* to the direction the wave travels. Think of a wave on a string: the string moves up and down (perpendicular to the direction the wave travels along the string).

* Longitudinal waves are the ones where the medium moves parallel to the direction the wave travels. Sound waves are a good example: the air molecules compress and expand in the same direction the sound wave is propagating.

Let me know if you have any other questions about waves!