Burr Coffee Grinders
Burr coffee grinders are more expensive than blade grinders but they have several advantages. The burr grinders grind coffee much more evenly that the blade grinders. They also heat up coffee less and don't build up as much static charge as their cheaper counterparts. The best coffee grinders around are low speed conical burr grinders.
How a burr coffee grinder works
When is comes to grinders, a burr refers to a metallic part that has sharp teeth cut into it. A burr grinder has two burrs with the cutting teeth facing each other. One of the burrs is fixed in place while the second burr rotates. This action grinds the beans in between the two burrs. The coarseness of the grind is adjusted by changing the distance between the two burrs. The closer the burrs are together, the finer the grind.
There are two different designs of the burrs. A flat burr grinder has a set of two ring shaped burrs. In a conical burr grinder the burrs are shaped like a cone. Both designs produce a consistent grind. However, with the conical burr grinder there is a larger grinding area given the same diameter. That allows using lower speed of rotation. This is the reason why conical burrs are often used in high-end low speed grinders.
High speed and low speed grinders
Coffee should be ground slowly. In fact you will get best coffee using a manual grinder - if you have the time and energy to use one. Grinding coffee at high speeds heats it up and that destroys some of the flavor. It also causes static charge to build up - annoying thing that causes the ground coffee to go all over the place and attach to anything in sight. So you really want to have a low speed burr grinder.
There are two different mechanisms used in low speed burr coffee grinders: the gear reduction and direct drive. With the gear reduction you get a high speed motor with a set of gears that reduce the speed of the motor. With the direct drive you get a powerful low-speed motor that drives the rotating burr directly. This mechanism is by far the best: these grinders are very quiet and you get very little heat and static.
The advantage of high speed grinders is the cost - they are cheaper than the low speed models. If a low speed grinder is out of your budget then you should get a high speed burr grinder. It will heat up the coffee more and it will be more noisy than the low speed grinders but it is still a big improvement over the blade grinder.
Other things to consider
Ways to dispense ground coffee
Burr coffee grinders come either with a doser or without it. Grinders with a doser are designed for use with espresso machines and they are often called dosing grinders or doser grinders. OK, so what is a doser? Doser is a special type of container that catches the ground coffee. It is rounded and it is separated into 6 segments (like when you cut a pie into 6 pieces). You can then dispense the ground coffee segment by segment directly into your portafilter. A segment usually holds about 7 grams of ground coffee - just the right amount for a single espresso shot. With some grinders you can adjust how much coffee each segment contains.
Grinders without a doser are referred to as doserless grinders or as non-dosing grinders. There are two ways these grinders dispense the ground coffee. One way is to dispense the ground coffee into an internal container that you can then take out. The other way is to dispense the ground coffee directly into a container you put under it (for example a filter basket or a portafilter). If you plan to use your grinder with a coffee maker rather than with an espresso machine then you should get a doserless burr grinder.
Ways to adjust the coarseness of the grind
The coarseness of the grind depends on the distance between the two burrs. There are different mechanisms that allow you to change this distance. What is important for you to know is the difference between the stepped adjustment and stepless adjustment.
The burr coffee grinders with stepped adjustment have several pre-set grind settings. The advantage of these grinders is that you can easily switch between different grind settings. That makes the stepped adjustment grinders very popular.
With the stepless adjustment grinders you don't get any pre-set settings - you can change the grind gradually to whatever coarseness you want. The stepless grinders are great if you are trying to fine-tune a grind for your espresso machine.
Cheap burr coffee grinders
Cheap burr coffee grinders (up to about $50) are usually not worth the money and they may not be much better than a good blade grinder. The burrs on the cheap models are not that well made and they are often housed in plastic. The plastic doesn't hold the burrs in place firmly enough which leads to uneven grind. The cheapest burr grinders usually produce quite a bit of dust and uneven particles. I would definitely advise against using them with an espresso machine.
Further ReadingCapresso Infinity Burr Grinder
Turkish Coffee Grinder
How to Grind Coffee