EM waves, as I shall call them are basically perpendicularly oscillating electric and magnetic fields propagating in space. The Oscillating fields oscillate transversely i.e. perpendicular to the direction of propagation.
Their velocity in a particular medium is determined thus by the electrical and magnetic properties of the medium.
In vacuum, they propagate with a velocity approximately given as
In presence of material medium their velocity is smaller than this and is given as