Grinders come in all sorts of different sizes, shapes, weights and styles. Even the teeth are different in certain grinders and will grind in different ways. The type of grinder you choose will depend on your needs and your budget, as grinders can range in price from very inexpensive to quite pricey. One needs to assess their needs and decide on which one fits their preferences best.
Regular 2-piece
Standard two-piece grinders come in a variety of shapes, styles and colours. Commonly, grinders are manufactured out of wood, metal, or acrylic plastic, and you can often find a two-piece grinder of any variety for quite cheaply. Some people prefer a two-piece grinder because it doesn't sift any of the finer material out and maintains the potency of your bud; however, they are often paired with a tray of some kind, as you will need to empty the product from your grinder before you can get to smoking.
3 piece
Three-piece grinders are, as the name suggests, a happy medium between a simple two-piece grinder and a more advanced four-piece. Three-piece grinders are fantastic because they help maintain potency by not sifting away sought-after material, but still offering a space for your product to collect, meaning you don't need additional trays or storage jars.
4 piece
Four-piece grinders are great for heavy smokers, as they grind your product, collect it below and sift some of the kief or Trichromes from your buds and collect them for you. Trichromes are a part of the cannabis plant that store most of the sought-after chemicals, such as THC or CBD, and a four-piece grinder allows you to access just the trichromes or keif, as we call it, once it builds up in the bottom. I often equate the bottom "keif catch" in a four-piece grinder to a tax-free savings account that only saves you nice big bills.
Electric Grinder
Electric grinders are wonderful for people who find it difficult operating a standard hand grinder or people who are grinding up such a quantity that a normal grinder just doesn't suffice. However, they rarely offer features like keif collection or portability. They range from a basic handheld to the fancy Otto grinder, which rolls nice cones. Electric grinders are nice for convenience and provide good grinding for those joints.
With all the choices out there, you have a good selection of different grinders for different uses. Use the knowledge you have gained, and choose one that suits your needs.