Batting Ratings determine the offensive skills, abilities, and ultimately the results that a batter will experience when he steps up to the batter's box or takes a lead at first base.
Discipline reflects how well the batter can recognize balls and strikes, and whether they are patient enough to swing at good pitches only. DI has a dramatic effect on walks and a lesser effect on strikeouts (usually on called strikes). High Discipline does not necessarily mean the player hits better - but it does mean their on-base percentage will be a lot higher. Also, Discipline makes them a little more (or less) likely to be hit by a pitch, since they are taking more (or less) pitches. An average player with high DI will typically walk a lot more and strike out a little less.
Famous players with high Discipline: Rickey Henderson, Joe Morgan
Contact reflects how well the batter can make contact when he decides to swing. Whereas Discipline has a big effect on walks and a small effect on strikeouts, Contact does the opposite - dramatically affecting a player's strikeout chances, and slightly affecting their walk chances. Contact has a very small effect on their ability to get a single or double (sometimes just making contact is enough), and has a slight effect on the Ground Ball/Fly Ball ratio (high contact batters will keep the ball down a little better). Give the average player a high CN and he's likely to come in with improved AVG and OBP. Bear in mind however, those improvements comes at a cost, as more outs in play will mean more chances for grounding into doubleplays.
Batting reflects how well the batter hits for average. This has the most pronounced influence over the ability to hit singles and doubles, and has a slight influence over the triple and home run totals. Batting also has a small influence over walks and strikeouts - the higher the Batting, the less strikeouts and more walks, though not nearly to the degree that DI/CN affect those outcomes. GB/FB ratio is also affected by this - again, presuming that when a player hits for average they tend to keep the ball down a little better. An average player with a high BA gets significantly better AVG and OBP. (Note that the increase in OBP is primarily due to the increase in batting average.)
Famous players with high Batting: Wade Boggs, Ted Williams
Slugging reflects how well the batter hits for power. If you want home runs, you want SL. Slugging basically converts singles to extra-base hits without increasing the overall batting average much. Think of a .330 hitter with 10 HRs and a .330 hitter with 40 HRs. Neither player had more hits, but the 40 HR guy simply had less singles (and maybe doubles) and more home runs. High Slugging also affects walks because pitchers try to pitch around power hitters more often. These players also tend to have more strikeouts because they miss the ball a little more often when swinging for the fences. Many high SL players have a low GB/FB ratio, since the home run shots that don't quite make it tend to be fly ball outs. The average Joe will see a marked increase in homeruns with high SL.
Famous players with high Slugging: Babe Ruth, Hank Aaron
Speed reflects how well and how fast a player runs the bases. SP is not just a measure of raw speed - it's a combination of speed plus baserunning instincts. Aside from the obvious influence on stolen bases, Speed will also influence the rate and success of a player taking extra bases, tagging up, and breaking up double plays. The average player will attempt to steal approximately every 1 in 16 chances. A player with SP=100 will steal about 1 in 2 chances.
Famous players with high Speed: Lou Brock, Willie Mays