When making coffee, one of the essentials is making sure the coffee is ground to fit the brewing method.
To start off, there are three types of grinders that most people use: a blade grinder, a conical burr grinder, and a flat burr grinder.
It is always best to avoid using a blade grinder, as they give the least consistent results, which is the opposite of what anyone would want, while the other two options can differ depending on the manufacturer.
Blade grinders: Are small cone shaped devices where you put the beans inside, and push a button, which causes blades start spinning around chopping up the beans, similar to a food processor. Because of how the beans will fly around inside, the end result will be smaller and larger particles. Also, the fast movement of the blades can create a lot of friction, and end up cooking the coffee beans after a short period of time.
Conical burr grinders: Burr grinders in general give a much more consistent particle size, as they consist of two plates, or in the conical case, cones, that are made of either ceramic or steel that rotate and break up the beans. The distance between the two plates can be easily set on most grinders, so the larger the gap between the burrs, the more coarse the coffee will be. For most conical grinders, the moving burr rotates much more slowly and better heat spreading than that of a flat burr grinders, which results in less friction.
Flat burr grinders: These machines are the most popular for industrial settings, as they are known for being able to grind large amounts of coffee pretty quick. Other than grinding faster, the results are usually the same as with conical grinders, but of course that depends on the company that made them. I use both styles of burr grinders just about every day when making coffee.
Steel vs Ceramic: As you mill the coffee it will create heat due to friction in the grinder, this heat will cause the oils and aroma to escape. Ideally you want to capture all of this aroma and oil in the final ground coffee so purists are switching to ceramic burr grinders as less heat is transferred to the beans as they are ground.
Hand milled coffee tastes nicer than most electrically milled coffee but it takes so long to grind is not a practical proposition.