Arc and glow discharges explained!
Electrons normally don't just move or flow from a conductor into a gas. Something has to make this happen. Explained below are ways for this to happen.
In a glow discharge, positive ions bombarding the cathode dislodge electrons from the cathode material. There is a substantial electric field near the cathode that accelerates ions toward the cathode to make this happen. The whole process tends to complicate itself, resulting in a double layer of glow around the cathode, thin dark spaces underneath and between these layers, and a more substantial dark space between all of this and either the anode or the main body of the discharge, whichever comes first. In neon glow lamps, the anode is so nearby that no main discharge body occurs.
"Neon" signs are longer, so a main discharge body occurs. Since these operate on AC, each end has a significant dark space only half the time, so these regions are a bit dim rather than dark.
There is generally a natural current density in the cathode process, generally around a milliamp to .1 amp per square centimeter, depending on the gases involved, the pressure thereof, and the cathode material. A glow discharge at this intensity is a "normal glow". Decreasing the current causes the cathode's glowing layers to cover only part of the cathode. In this case, the glow often moves around, causing a flickering effect.
If the current is more than enough to cause the cathode to be covered with glow, (or if the glowing layers are forced into a thinner layer of space than they normally use), abnormal glow results. The voltage drop of the cathode process (this voltage is known as the "cathode fall") will be higher than normal. This causes ions to bombard the cathode harder than usual. This increases "sputtering", or dislodging of cathode material atoms. Sputtering effectively "evaporates" cathode material and often causes darkening of the lamp's inner surface.
Sputtering occurs more easily at higher cathode temperatures. It is generally recommended to neither have significantly "abnormal" glow nor significant temperature rise in the cathode, and especially not both of these combined.

The cathode fall of normal glow is usually 50 to 90 volts for neon, argon, krypton, xenon, or mixtures including significant amounts of any of these gases. Some metal vapors may have somewhat lower cathode falls. Nitrogen and some other gases have high cathode falls usually near or even well over 100 volts.

The cathode process in most HID lamps and fluorescent lamps is the thermionic arc. In this process, at the proper high temperature, some material in the cathode fails to keep a grip on its electrons. Therefore, electrons simply flow from the cathode to the gas. The cathode fall is usually around 10 volts, and the heat dissipated in this process keeps the cathode hot enough to let electrons flow from it to the gas.
The current density at the cathode process of a thermionic arc is generally in the tens or hundreds of amps per square centimeter of active cathode surface, but can occaisionally be as low as around an amp per square centimeter if a heat source other than the arc heats the cathode.

Another arc process is the cold cathode arc. In this process, ions bombard the cathode material and dislodge electrons from it. This seems similar to the glow discharge, but the effect is quite different. The current density in the cathode process is usually hundreds or thousands of amps per square centimeter. The cathode fall is usually near the ionization potential of the cathode material or the main active gas ingredient, whichever is lower (for a minimum) to twice whichever is higher (for a maximum). Substantial sputtering may occur, especially if the cathode is hot. Cold tungsten is usually reasonably tolerant of this, permitting the use of this process in xenon flashtubes.

An arc is often not entirely thermionic nor cold-cathode, but one of these processes is usually dominant.

If a hot-cathode lamp is underpowered, the cathode is not as able to emit electrons by the thermionic process, and significant cold-cathode arc process may occur. This can cause excessive sputtering. Starting a hot-cathode lamp also results in some of this as the cathode warms up. Overpowering a hot-cathode lamp can simply overheat the cathodes.
Because of this, it is generally advised to start fluorescent and HID lamps as infrequently as practical and to neither overpower nor underpower them. This makes it difficult to dim fluorescent and HID lamps significantly without being hard on their cathodes.
There are some special dimming ballasts for some fluorescent lamps. These dissipate power into the cathodes to maintain a workable thermionic process when these lamps are dimmed. It is recommended to only dim fluorescent lamps with appropriate ballasts, and to use these dimming ballasts only with the lamps they were designed to safely dim.