The image below is the Bohr model. The different "levels" n=1,2,3,4,... are called excitation states or levels (~energy levels). What it is saying is that the electrons orbit in eliptical orbits around the atomic nucleus. The force that's keeping the electrons in their orbit is magnetic and not gravitational. If the forces were gravitational over time electrons would collide with the nucleus. What the (delta) E = hv is saying is that if an electron is in an excited state, that is something like a photon (light) for example has "pushed" one electron away from the nucleus. It will return to it's normal state of excitation, releasing the energy it absorbed as a photon. Note that in every atom the energy required to "push" an electron to a higher state of excitation is different and there is a difference between the states.
About his discoveries you can find more information on wikipedia.