It's a lot of common sense really...

Seasonal Yoga is yoga as you probably already know it, in fact unless your teacher highlights it and brings your attention to it, you may not even know you're at a Seasonal class. 
Seasonal Yoga takes traditional yoga asanas but incorporates the elements of meridian lines and TCM, along with looking at the natural world around you. An example is in the winter, nature starts coming in on itself, hibernating. Seasonal practise in the winter would mimic this process, allowing our bodies to take an easier practise, soothing ourselves and storing up our energy for the months ahead. In the summer months our practise is much more driven by generating heat within our bodies and awakening our internal fire and drive. 
It is a dynamic style of yoga which changes with the energies of the seasons, bringing you back in line with the energy surrounding you in your every day life. It can be life changing!