Deep neural networks (DNNs) are a powerful type of artificial intelligence (AI) that can learn complex patterns from data. This text explains the key components of DNNs, such as neurons, activation functions, and layers, and how they work through forward propagation and backpropagation. The text then explores different types of DNNs, including convolutional neural networks (CNNs), recurrent neural networks (RNNs), generative adversarial networks (GANs), and transformers, highlighting their strengths in specific applications like image recognition, natural language processing, and autonomous driving. It also discusses current trends in DNN research, such as explainable AI (XAI), foundation models, edge AI, and energy-efficient deep learning. The text concludes by highlighting the significant impact of DNNs in healthcare, and the ethical considerations surrounding AI bias mitigation.