Scattering of electromagnetic energy takes place when the waves of light, heat, and ultraviolet radiation interact with the particulates found in the atmosphere and water. These particulates might be in the form of dust, gases such as water, or pollution.
As electromagnetic waves hit these particulates they are bounced off in random directions. In other words, they are scattered. The direction that they are scattered depends on the shape and size of the particulates.
Smaller wavelengths, such as blue light, tend to scatter more than longer wavelengths. This is why the sky looks blue. What you see above your head in all directions is blue light being scattered from one particulate to another.