Defensive Ratings determine the skills, abilities, and ultimately, results, that a fielder will experience whenever he puts on his glove and takes the field.
Skills are measured on a scale of 1-100, with 1 being the worst and 100 being the best. It goes without saying that a higher rating = more skill in that rated area, but it's also important to know that the scale is not linear
. The difference in ability gained between 50 and 60 is less than the difference in ability gained between 60 and 70. Starting with ratings of 50 or higher, the higher the number, the larger the difference. The difference between a 95 and a 98 may be similar in scale to the difference between a 50 and a 70.
Range measures how much ground a player can cover on the field. If you've got great range, you get to more batted balls than other players do. Translation: more potential outs. Consider a centerfielder with great range - he'll catch more fly balls than a lesser centerfielder. Range up the middle (SS, 2B, CF) is typically the most valuable.
Famous players with high Range: Willie Mays, Ozzie Smith
"Soft hands" - can you field the ball or not? A player with a good glove makes fewer errors. One funny consideration is high Range and low Glove - you get to more balls in play, but fumble more of them. An interesting tradeoff. GL impacts the ability to field the ball (once you get to it), transfer it to your throwing hand cleanly, and throw on target to the desired base.
Famous players with high Glove: Brooks Robinson, Ryne Sandberg
A combination of how hard, far, and accurate a player can throw. For outfielders, this has a significant impact on the frequency and success rate of runners trying to tag up and take extra bases. For infielders, this can impact the success rate of double plays. Catchers, of course, need a good arm to throw out baserunners trying to steal. In all cases, AR can also contribute to errors, though the majority of errors are caused by the Glove rating. Note that AR does not have any influence on pitch speed!
Famous players with high Arm: Dave Winfield, Ivan Rodriguez