In my humble opinion, a grinder in one of the best investments a frequent smoker can make.
Grinders speed up and simplify the process of separating dry herbs, and make it much easier to conserve your herbs when they are finely ground. When pulling herb apart by hand, pollen and other plant matter can stick to your fingers, reducing its potency. However, when herb is ground, it can be packed more tightly into a bowl or rolling paper - meaning the herb is exposed to less oxygen, and therefore will burn less before you inhale and increase the airflow. This means that you are given more control over how quickly (or slowly), and smoothly it burns. Using herb in this manner, especially through a water pipe or bubbler, can greatly help you conserve your stash. A grinder can even act as a portable storage container for small amounts.
Most grinders are either metal - mostly aluminum - or plastic, and have at least two interlocking pieces that form an upper chamber and lower chamber. The upper chamber contains an array of teeth that, when the two pieces are twisted and move opposite of one another, slice and grind up any dry herb inserted inside. The ground herb then falls through the small holes between the two pieces and lands in the lower chamber. The number and shape of the teeth varies from brand to brand - this really only factors into personal preference, as each will provide varying degrees of grinding.
Many grinders have more than just the two chambers, such as a four piece grinder, which has a bottom chamber separated from the others by a metal mesh screen. This screen is meant to catch pollen that falls off the herb during the grinding process and store it in a separate chamber so it can then be used to your discretion. There are even five piece grinders, which have a second, even finer mesh screen to further separate the ground material.
It is common practice to place a coin or other small, weighted object inside of the grinder to help to shake any built up material off, and grinders need a small amount of maintenance and cleaning to prevent them from getting clogged.
Grinders come in many sizes, ranging from 1.5” in diameter to 3” and above. The size doesn’t factor into the function of the grinder - it all depends on how much herb you would like to grind up at one time. Some grinders also come equipped with other specialty features, such as vibration functions, but they all work improve your smoking experience.