Turbulence is essentially a random phenomenom.
Windshear is, at it's simplest, two layers of air moving at different speeds, hence with a "shear" or tearing layer between them. It can also be a local up or downdraft, and associated in/outflow. Regardless, it's a pseudo-stable condition - if there was a downdraft of strength X 30 seconds ago, it's probably still there, and of similar magnitude.
In practice, one rarely sees windshear without some level of turbulence; the academic windshears used for training and system design therefore comprise both a pure shear model and a turbulence model.
AC120-41 contains some discussion of the modelling approach used.