The main difference is that TensorFlow is a framework for machine learning, and OpenCV is a library for computer vision. It can be a good start to check the link below to get a grasp for the difference between framework and library: What is the difference between a framework and a library?
You can do image recognition with TensorFlow. Though it is suited for more general problems as well, such as: classification, clustering and regression.
I guess people downvoted because this question might be more relevant to: https://datascience.stackexchange.com/