Convolutional neural networks (CNNs) are a type of neural network used in image recognition and processing. CNNs use convolutional layers that apply filters to input volumes to extract features at different spatial locations. Backpropagation is used to train CNNs by propagating errors backwards. CNNs have been successfully applied to tasks like ImageNet classification, object detection, and image captioning. Convolutional layers are the core building blocks of CNNs, applying filters to input volumes to produce activation maps as output.