All forms of radiation (or light) can be thought of as consisting of waves. Therefore, we can characterize electromagnetic waves by their WAVELENGTH.
Let's use an analogy to the waves on the ocean. Think about what happens to a small boat bobbing on the ocean. If the distance between the wave crests is large ( i.e. the wavelength is long), then the boat bobs up and down less often ( the frequency is low); if the distance is small ( i.e. the wavelength is short), then the boat bobs up and down more often ( the frequency is high).
Radio waves have much longer wavelengths and lower frequencies than do visible light waves. In contrast, X-rays have much shorter wavelengths and higher frequencies. Likewise, red light has a longer wavelength than blue light.
We can also think of light as consisting of particles or packets on information that we call photons.