Construction Materials (working from the inside out):

Cork Center: 1/2 ounce and 2.86 to 2.94 inches.
Black & Red Rubber Layers: 7/8ths of an ounce each.
4-ply grey wool: 121 yards (brings weight up to 2 7/8 ounces).
3-ply white wool: 45 yards (brings circumference up to 8 3/4 inches).
3-ply grey wool: 53 yards (brings weight to 3 11/16 ounces and circumference to 8 7/8 inches).
Cotton yarn: 150 yards (brings weight to 4 5/8 ounces).

Those materials are put together using a high tension sewing machine then its all sealed with a coat of rubber cement. The cow-hide (horse-hide was used until 1974) is then sewn on using 108 (or 216 depending on the source & how you choose to count a “stitch”) stitches from red cotton.

Source:  http://www.baseball-fever.com/showthread.php?3886-Inside-A-Modern-Baseball