According to classical theories, electrons could have any energy from an infinite range of energies. This can be represented in an energy-level diagram such as the one on the left.
 

However, according to Planck's quantum hypothesis, the energy of the electron is "quantized" ; that is, it can only have certain discrete values.  This is represented by the diagram on the right.

In fact the energy levels in atoms, the simplest of which is hydrogen, are not equally spaced. Evidence for the existence of these levels comes from the fact that atoms of a gas emit light of only certain discrete wavelengths. The visible lines in the hydrogen spectrum, known as Balmer lines, provide a means of calculating these energy levels.