Twelve curated topics covering what every AI engineer uses day-to-day: confusion matrix, accuracy/precision/recall/F1, Precision-at-K and Mean Reciprocal Rank and NDCG for retrieval, BLEU/ROUGE & perplexity for generation, calibration & ECE, AUC-ROC vs AUC-PR, drift detection with KS/PSI, regression metrics, business metrics, A/B testing & statistical significance. Beginner-friendly framing with three worked examples each.