The process of grinding coffee beans is pretty essential to brewing. The only type of coffee I know of that does not use ground beans, is a type made for cupping.
There are tons of different ways to grind coffee, such as an electric grinder, hand grinder, or even mashing the beans between a couple rocks if that’s your thing. While some grinders are better than others, it is better to use freshly ground coffee from a not-so-great grinder, than it is to use coffee ground a while ago.
A lot of commercial coffee grinders will promote the speed at which a specified amount of coffee can be ground per second. Though, speed is not as important as many people think.
When the coffee beans are ground quickly, there is more heat buildup within the burrs, which could burn the coffee beans (or evaporate off the essential oils) and detract from the flavor, especially if the grinder’s burrs are dull. It is best to always make sure that the burrs are sharp, as when they are dull, they will just crush the coffee instead of “cutting” through it, which builds up more heat.
Ceramic burrs are less likely to get warm compared to metal ones and they have a longer life span so ceramic is the way to go.
Hand grinders, especially higher end ones, grind coffee just as well as an electric grinder would. Because hand grinders are hand cranked, the operator can easily speed up or slow down how fast they are going.
Though, some hand grinders can have less consistency in the grind when it comes to more coarse grind levels, such as for a French press. To combat this, there are products on the market that claim to stabilize the shaft in some of these models, which will result in a more consistent grind.
Another con of hand grinders is that if you have to grind a lot of coffee, you may as well cancel your gym membership. While there are ways to attach a power drill to the crankshaft of the grinder, be wary not to use too much power (a friend did this and destroyed the burrs in his grinder causing fragments to contaminate the coffee).
Electric coffee grinders are a thing of wonder really, and come with various functions. Some have built in timers for the duration of time you want to grind coffee for, others have an almost endless amount of grind possibilities with two built in adjustment rings, as opposed to just one.
For the most part, grinders gain features and produce a more consistent grind as they go up in cost, and some grinders are meant for a specific brewing style, which is more often than not, espresso. One of the things that a lot of electric grinders struggle with, is being able to grind good espresso and be able to grind consistent coffee for a French press.
Because these two brewing styles require grind levels on opposite ends of the spectrum, it is difficult for a more specialized grinder to grind that coarsely while maintaining a consistent grind.
Once coffee is ground, it is good to consume it within 24 hours, but it is best to use the coffee right after it has been ground, to ensure no loss of flavor.