What are the letters in front of the exercises on my program for?
Letters which appear in parentheses in front of an exercise indicate that the exercises which share the same letter should be performed as "supersets". Supersets are two exercises performed in a row without stopping. For example, if Bench Press is combined in a superset with Chin-ups, you would do set-1 of bench press, immediately (no rest) followed by set-1 of chin-ups. Rest in between and then do set-2 of bench press immediately followed by set-2 of chin-ups, and so on until all sets are completed for both exercises.
It takes a while to build up the endurance necessary to do a lot of supersets, but this kind of conditioning developes in time if you keep working at it. Supersets can be used to train 2 different body parts (as in the bench press/chin-ups example above), or you can do two exercises in a row for the same body part. You will be surprised how a muscle which seems totally fatigued will still have a lot of strength remaining if you demand that it perform a slightly different movement.
Supersets are especially good for training opposite body parts simultaneously - chest and back for example. This gives you a tremendous pump as you perform the alternating pushing and pulling movements, yet gives each muscle group involved the minimum chance to rest and recuperate.