Grinding is the fracturing of coffee bean particle cells. Its purpose is to increase the amount of coffee solids exposed to the extracting liquid.
Quality espresso requires an exceptionally fine grind for numerous reasons.
I recommend you invest in the best grinder you can afford, even if it means you have to buy a cheaper espresso machine. A mediocre grinder under heavy use can damage flavor with excessive heat and can prevent even extraction by producing clumps, too many fines, or poor distribution of grounds in the basket. No espresso machine, no matter how impressive, can (yet) compensate for the problems created by poor grind quality.
The single most important feature of a grinder is sharp burrs. This cannot be overemphasized. Sharp burrs create less strain on a grinder's motor, generate less heat, produce fewer fines, and offer better particle size distribution. Because it can be expensive to regularly purchase new burrs, I recommend you find a local machine shop or grinder manufacturer willing to resharpen dull burrs. Burrs can be resharpened several times before they need to be replaced.
A home barista who rarely pulls more than two or three shots in an hour will probably not notice much difference in the performance of various professional-quality grinders. A home barista also has the luxury of using a time-consuming method such as the Weiss Distribution Technique to compensate for poor grinder performance. (See "Grooming" in Chapter 2.) Therefore a home barista can achieve consistently excellent results with any professional grinder of reasonable quality.
On the other hand, a barista who works in a café and frequently pulls several shots in quick succession needs to be more careful when choosing a grinder. A professional barista needs a grinder that facilitates even distribution and does not overheat the grounds when under heavy use.
Criteria for Evaluating a Grinder
Grinding Systems: Pregrinding Versus Grinding To Order
Adjusting the Grind