The smaller end allows the fly to land on the water more softly. Leaders are classified using an X system. 0X being the largest and having a 12 lb. test factor. 8X is the thinnest and is 1 lb. test. Surface flies can either be dry flies, which imitate an adult insect, or poppers.
Poppers imitate anything from injured animals to frogs.
Sub-surface flies can be wet flies or nymphs. The main difference between the two is that nymphs have wings. If you are at a trout park, ask the attendant what the fish are biting on that particular day. Bait can always change from day to day. These are breif descriptions of some begining casting techniques to use.