Classical physics incorporates the assumption that if we are clever enough and careful enough we can continue, indefinitely to improve the precision with which anything can be measured. Werner Heisenberg discovered that in the realm of photons and electrons, this assumption does not hold up. What is called the Heisenberg Uncertainty Principle may be stated as follows:

It is impossible to know simultaneously both the exact momentum and the exact position of an electron (momentum is mass x velocity - it expresses not only the tendency of a moving body to keep moving, but also, since velocity is a directional quantity, to maintain the direction of its motion).

In summary, All electromagnetic radiation is characterised by its l and n and displays the wave properties of interference and diffraction. On the other hand, the energy of radiation is quantised according to E = hn . The energy of electrons is also quantised by E = hn . Electrons and similar small particles obey quantum-mechanical laws and exhibit the wave properties of interference and diffraction. It is not possible to know simultaneously both the position and momentum of such particles.

Information provided by: http://www.bpx.cov.ac.uk