You need to match your grinder to your brewing method. Check out the Grinder Comparison Chart to identify the pros and cons of each model and match the grinder to your purposes.

Grinding essentially exposes the surface area of the bean to water - and water is a solvent...the longer and more contact it has with the coffee, the more it will extract. The degree of roast and the freshness of the beans impact how the beans will grind too; for example, a dark roast is more brittle and will crumble more; a dark roast can tend toward overextraction for this reason. In some brewing methods, you control extraction time only indirectly; for example in a standard filtercone your only way to slow extraction time is to use a finer grind. Extraction time will vary based on the two other factors - say your grind is a bit too fine because you have a dark roast but you live at altitude and water boils at a lower temperature.

Check out the Brewing Fundamentals page for more on the interrelation of grind, dose and extraction time.