First two answers to your questions must have gave you some good understanding of Energy, Spectra & some history of early Quantum Mechanics.
Please read them again before you read my analogy.
I'm directly moving onto analogy without wasting "Space" & "Time".
In Classical systems, the movement of energy is like an movement upwards on a Ramp.
In Quantum systems, the movement of energy is like movement upwards on Stairs.
On a ramp, the moment is straight and continuous. The higher work done - the higher you go.
On a stair case, the moment is not continuous. Each step you take for a higher planes requires a "unique" value of work done to reach it.
Consider Photo-Electric effect : (If you don't know what it is, Wiki it)
Imagine you're using Red light to strip of Electrons from a Iridium Metal. Iridium is one of those which requires almost close to 6 eV to strip of electrons from it's surface.
Among the visible region Red is the one with very low frequencies - naturally low energy. If we irradiate the Red light on the Iridium metal - at first, as expected - the electrons don't strip off. If we keep increasing the "Intensity" of red light i.e. giving more & more Red light on the metal - From Classical idea of Energy, we'd expect -- energy must keep increasing in some systemic way and eventually electrons have to strip off.
Here's exactly where Classical Mechanics ends & Quantum Mechanics rise.
From first answer by Mr.Gaurav, you can clearly understand that Red light energy can only take unto a certain level of stairs. After that it stops. However, intensity you provide after that ,.. it doesn't matter. It reaches it's maximum & start decreasing there after.
Look at this Graph - Each Wavelength of varying Intensity with Temperature.
You can clearly how each color (wavelength) reaches it's maximum upto a point & starts declining after.
Classical view predicts this :
As you keep increasing the intensity, the energy (Temperature) keeps building up continuously. That's the not the case in reality. Each frequency has it's own maximum capacity.
So, as per Classical mechanics, as you keep increasing the intensity - the energy builds up & you move onto higher & higher level on the ramp. There are no such things as levels.
Please keep in mind, all this analogy is just to understand the idea of Quantization - the actual idea of Quantization is bit more complicated than this.
In fact, even Max Planck was skeptical of this very idea to the end. He thought of this Quantization - as some mathematical trick to solve the Ultra-Voilet Catastrophe of Rayleigh-Jeann Law in explaining Blackbody radiation.
It's like using Abacus to Add numbers when we were kids. At that age, it's the best we can operate & perform additions. As we grow up, we move onto much sophisticated methods & tools.
Same with the idea of Quantization or entire Quantum Mechanics for that matter. Right now, it's like Abacus for us to understand Atomic world. Probably in time, we move onto much higher & sophisticated ideas.
I hope it helps.
PS: For better understanding, refer these topics : Black-Body Radiation, Ultra-Voilet Catastrophe, Equipartition Theorem of Statistical Mechanics & Max Planck early contributions to Quantum Mechanics.