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The atomic nucleus is a tiny, dense region at the center of an atom, where most of the atom's mass is concentrated. Both the Rutherford and Bohr models recognize the nucleus as comprising protons, which have a positive charge, and neutrons, which have no charge. This central cluster is contrasted with the cloud of negatively charged electrons that surrounds it.

The Rutherford model, proposed in 1911, was revolutionary because it introduced the concept of the nucleus based on the famous gold foil experiment. This experiment provided evidence that atoms consist of a dense nucleus around which electrons revolve. Before this discovery, atoms were thought to be indivisible particles or a 'plum pudding' model where electrons were embedded in a sphere of positive charge.

Bohr's model, presented two years later in 1913, retained the idea of a central nucleus but added new concepts about how electrons exist around this nucleus. The finer details of the atomic nucleus, such as the existence of neutrons, were not yet known at the time these models were proposed, but the presence of a positively charged core was a critical aspect acknowledged by both.

The concept of quantization is a fundamental principle introduced by Bohr, which is absent in Rutherford's model. According to Bohr, electrons can only occupy specific energy levels, called quantum states, and can transition between these levels by absorbing or emitting energy in discrete amounts known as 'quanta.' The quantization of energy levels explains why atoms emit or absorb electromagnetic radiation at specific wavelengths.

In the Bohr model, these quantized energy levels are visualized as distinct orbits around the nucleus, each with a fixed energy value. When an electron jumps from a higher to a lower energy level, it emits a photon of light with an energy equal to the difference between the two levels. This idea was not only revolutionary for understanding atomic structure but also crucial for developing quantum mechanics.

Rutherford's model, on the other hand, did not touch upon the concept of quantized energy states. His model included electrons orbiting the nucleus but could not explain why these electrons do not spiral into the nucleus due to radiating energy, a predicament resolved by Bohr's quantization idea.

Electron orbits are the paths that electrons are predicted to follow around the nucleus of an atom. In Rutherford's model, these paths were not well-defined, and electrons were thought to circle the nucleus similar to planets orbiting the sun. Although this offered a visual picture, it failed to provide a stable configuration for the atom, as classical physics would predict that the electrons would lose energy and collapse into the nucleus.

Bohr's improvement on this concept resulted in discrete orbits for electrons, which correspond to certain allowed energy levels. In Bohr's model, an electron orbiting in one of these fixed orbits does not radiate energy, therefore it does not spiral into the nucleus. This was a significant leap in atomic theory as it provided a reason for the atom's stability, which Rutherford's model could not.

By suggesting that the electrons could only 'jump' between these fixed orbits and not exist in between, Bohr introduced the idea of quantum leaps or transitions. This aspect of his model was vital for the later development of quantum mechanics and helped explain a variety of atomic phenomena, including the spectral lines of different elements.