[maudineormsby]

Quick explanation:

Fines are the very small coffee particles created by the grinding or crushing of the bean. They are generally undesirable as for even extraction of the coffee you want uniformly sized coffee particles. Fines also tend to end up in your cup and can make the brew gritty.

Extractions can mean a few things but in a nutshell, it's the amount of flavor compounds in the bean that end up in your brew. If you say '20%' extracted you mean that 20% of the weight of the coffee ended up taken out by the brewing process (I believe you brew then dry the grounds and weigh before/after).

Slight differences could be the difference between bold flavor and over-brewed bitterness. Also different coffees will want different extraction amounts depending on their flavor profile and such; as a general rule most people use too little coffee and overextract it, thus getting all the bitter compounds from the coffee and thinking it tastes bad.