TL;DR
OpenCV is the standard open-source library for developing computer vision applications, providing various functions such as image processing, object detection, and face recognition.
Key features
Image and video processing: filtering, transformation, feature extraction, etc.
Object detection and tracking: Haar Cascade, HOG, deep learning-based detectors
Machine learning: built-in algorithms such as SVM, KNN, random forest
Camera calibration, 3D reconstruction
Deep learning module: load and inference TensorFlow, PyTorch, Caffe models
When to use it
Computer vision research and prototyping
Real-time video processing systems (robotics, autonomous driving, security)
Any application requiring image analysis