A basic overview of the training programs and workouts in Zwift is at whatsonzwift.com
You can also import workouts and workout programs on Training Peaks into Zwift. You have to pay extra for those plans.
As a personal note I'm currently in my first training program in Zwift, it is a 12 week program. It is giving me a fantastic workout and I expect to see gains when it's over. But for me as an individual that enjoys social rides, 12 weeks doing just workouts is a bit much. My next program will be prolly 4 weeks.