What is the difference between strabismus and stereopsis?

Terminology. In strabismus the eyes present 2 different images to the brain which would result in a double image. Over time the brain may turn off one of the images & keep the best one. In stereopsis the eyes present the same image to the brain separated by the distance between the eyes. This allows you to have depth perception.
Strabismus. Strabismus means an eye muscle disorder that can be intermittent or constant. If it is constant there is no depth perception (or stereopsis).
Two different issues. strabismus - eyes to not maintain proper alignment
stereopsis - depth perception.