A multi cell battery in steady use goes through charge and discharge cycles. Charging and discharging is the process of putting CURRENT (amps) into the battery and taking CURRENT out of the battery. The amount of charge transfered in each case is measured in amps X hours or amp-hours.
You will notice the word "voltage" did not occur in the first paragraph. Voltage is used to push current into a battery or draw current out of a battery. Each cell has its own voltage that is added up in 6 cells (for a 12 volt battery) to give the battery voltage. All the cells contribute equally to the current flowing out - they have to because they are in series and the identical current has to flow in all of them and they share equally in the current coming in - again they have no choice independent of the individual cell voltages.
A battery with cells in series is like a chain and it is as strong as the weakest link. The whole battery is considered discharged when the weakest cell is discharged. If all the cells have identical characteristics then they should all reach the discharged state about the same time.
During charging the charger can't monitor and charge the individual cells. They all get charged equally and the charger monitors the total voltage to determine if they are fully charged.
But unfortunately they can't make batteries with "identical" cells - there is always one that is weaker and will be discharged (or charged) before the others. This means the unused capacity of all the other cells is wasted and the battery capacity is diminished to the weakest link. In fact that weak link will cause charging to stop before all the tougher cells are fully charged and their capacity is also reduced.
So EQUALIZATION is the process of forcing excess current into a battery that looks like it is already fully charged. This current will OVERCHARGE the weaker cell(s) causing them to bubble but that extra current will finish charging all the other cells that have been starved. When all 6 cells are gassing then no more equalization can occur and all cells are fully charged. An equalization charger can't check if they are all gassing so it usually just overcharges for a fixed time and assumes they have all "EQUALIZED" to full charge.
In this process, stirring up the electrolyte with the bubbles helps to destratify any layer concentrations and move any debris (sulfate) from between the plates.
With lead acid batteries, equalization is done at about 16 to 17 volts. Unfortunately this will do permanent damage to an AGM or GEL style battery since the bubbles will displace the electrolyte and form an insulation barrier between the plates. Voltages over 14.2 should be avoided with these batteries. While they have some capacity to re-absorb the bubbles, damage is usually permanent. They can be equalized but it is a very slow process, feeding in a trickle current that overcharges but not fast enough to cause bubbles over a period of days instead of hours. Monthly equalization is a typical schedule for batteries in daily use.
Sulfation in batteries is usually a symptom of DISUSE. In a battery in regular use with significant charge and discharge cycles sulfation is rarely a problem. I'm not knowledgeable enough of the chemical process of sulfation but sufficient to say you can think of it as crystals forming on the plates that reduces the area available for supplying current.
In particular I've found that batteries in long term float conditions such as at a dock with a charger maintaining float voltage, the tiny charge and discharge cycles can cause serious reduction in life. I don't know if that reduction is due to sulfation or just over use of just the surface of the plates making them spongy but read the article at How to protect your battery from "Short Cycle" damage. for information on how to save your expensive battery bank from "short cycle" damage.
When desulfators came out I read the patent claim(s). It read like science fiction and I was very dubious of their claims. The patent indicates that the sulfate crystals have a uniform size and if you apply a high frequency energy pulse to them and hit their resonant frequency (about 3+ MHz from memory) you can break them up just like the wine glass in the Memorex commercial from years ago. However despite my skeptical impression it is hard to deny the wealth of accolades from those that have used them.
So EQUALIZATION and DESULFATION are entirely different processes.