Some blades have vastly more teeth per inch than others, from as little as one to more than 24. A blade with only a single tooth per inch will result in a rough cut for most materials. A very finely toothed blade has the potential for a very clean-cut, but this is not always the case.
If one tries to cut wood with a blade that has 24 teeth per inch, the grooves in between each tooth will fill up with wood chips, and the blade will not be able to cut properly. Somewhere in the middle is best for most projects.