Particles in a medium carrying a transverse wave move perpendicular to the direction the wave travels.

Here's a way to visualize it:

* Imagine a rope tied to a wall. If you shake the rope up and down, you create a wave that travels horizontally along the rope.

* The particles (rope fibers) move vertically (up and down), but the wave itself travels horizontally.

This is in contrast to longitudinal waves where particles move back and forth in the same direction as the wave travels. Think of a slinky being compressed and stretched!