FaceAPI is a client-side and server-side library that runs TensorFlow.js models to perform face detection, face landmark detection, face recognition, and prediction of age, gender, and emotion from images or video frames. It wraps pre-trained models and provides an API for tasks like matching faces to known identities, tracking face rotation, and extracting face descriptors for comparison. The library works entirely in the browser without server-side inference or in Node.js environments.