Rami Arieli: "The Laser Adventure" Chapter 1.1.1 page 1
1.1.1 Electromagnetic Radiation in vacuum
Electromagnetic Radiation is a transverse wave(2), advancing in vacuum at a constant speed which is called: velocity of light.
All electromagnetic waves have the same velocity in vacuum, and its value is approximately:
c = 300,000 [km/sec] = 3*108 [m/sec] = 186,000 [miles/sec]

One of the most important parameters of a wave is its wavelength.

Wavelength (l) (Lamda) is the distance between two adjacent points on the wave, which have the same phase. As an example (see figure 1.1 below) the distance between two adjacent peaks of the wave.

You can play with an interactive demo of the meaning of wavelength, to understand how a wavelength can be measured at any point on the wave. Just click on:
Wavelength demonstration Applet

In a parallel way it is possible to define a wave by itsfrequency.

Frequency (n)(nu) is defined by the number of times that the wave oscillates per second (The number of periods of oscillations per second).

Between these two parameters the relation is:

c = l * n

From the physics point of view, all electromagnetic waves are equal (have the same properties) except for their wavelength (or frequency).
As an example: the speed of light is the same for visible light, radio waves, or x-rays.