A grinder is necessary if you want to crush your herbs into a smokable substance. Small pocket grinders are usually based on a rather simple method. They often consist of two parts with teeth that grind the material inside through constant rotation. These very simple grinders don't have an extra stash compartement.
Grinders – from different materials
Common wood grinder with metal teeth
Most low priced grinders that you can get online are made from acrylic. They are light weight, rather cheap as mentioned above, and easy to use.
If you need a more heavy duty grinder, then you should get a wooden or metal model. They are more durable. The metal grinders often have several compartments and sometimes a filter system for pollen crystals and a stash chamber. Yet, they may get pretty heavy. In this case, you might opt for a lighter aluminum grinder.
Wooden grinders can look like a piece of art, especially when they are beautifully crafted with a nice finish.
If you abhor any kind of exercise, then the battery powered grinders might be made for you. Just push a button and the grinder does its work.