HST - Hypertrophy specific training, usually has workouts that are in the higher rep ranges (8 - 12 reps) with shorter rest times (0 - 60 seconds). Basically, the main goal of this type of training is making the muscles bigger. Strength is usually not the main concern here.
If you stick with the same routine, rep range indefinatley you will adapt to it and gains will stop.
Hypertrophy is going to occur within the first 36-48 hours after a workout. The rest of the week is mostly spent not moving backwards. If you hit the muscles again after 48 hours, you can provide stimulus for more growth without significantly hampering recovery.
Also, the volume per exercise per workout session is only one or two sets, because more than that might just be a waste of time. The reduced number of sets allows you to get in a full body workout in just one session, and results in about the same total number of sets per week per muscle group.
The web site goes into more detail (it incorporates other ideas, like progressive load).
Not everyone agrees with HST principles - many feel strongly that muscles need a full week of recovery, so the idea of hitting them every other day is somewhat controversial. I just got started on HST myself, so I can't speak from first hand experience, but it seems like the principles are well supported by research.
http://www.hypertrophy-specific.com/hst_index.html