I was going to write this blog myself but upon opening this months 220Triathlon magazine Dan Bullock of Swim for Tri had done it for me! Dan breaks the session down into 1. Warm-up, 2. First sub-set, 3. Main set, 4. Second sub-set and 5. Cool-down. He then goes on to describe different types of swim session including: speed, interval, heart rate based, negative splits, even split and mixing strokes. I recommend you take a look.
220Triathlon is a good read this month. Tim (LFTC Coach)