### Summary

A Convolutional Neural Network (CNN) is a type of deep learning algorithm commonly used to analyze visuals. It’s particularly good at recognizing patterns in pictures, making it a key tool in image and video processing tasks.

### ELI5

Imagine you have a big picture made of tiny squares (pixels). A CNN is like a detective, first noticing small details (e.g., color, edges), then assembling these clues layer by layer to understand bigger objects or scenes (like a tree, a face, or a sunset).

### In-depth explanation

A Convolutional Neural Network (CNN) is a type of artificial neural network designed to process structured grid data, such as an image. It’s based on the idea of convolution, a mathematical operation permitting efficient information processing over neighborhood regions.

A CNN is distinguished by its multi-layer structure. The input layer is where raw pixel data are fed into the model. The convolutional layer applies a set of filters, each extracting a specific detail like edges or textures. The purpose of convolution is feature detection.

Next, the output from the convolution layers is forwarded to Rectified Linear Units (ReLU) layers, which introduce non-linearity to the CNN. The ReLU layer, introduces the mathematical process of max(0, x), replacing all negative pixel values with zero.

Following the filtering, the data undergoes a pooling or downsampling layer. The pooling layer reduces the spatial dimensions (width and height) of the input, making the CNN more computationally efficient. There are different types of pooling, the most common is max pooling, where the maximum value is taken from a window of pixels.

The process repeats, with the network learning more complex features and structures at each level. After a series of convolution, ReLU, and pooling operations, the finalized grid structures are flattened into a vector in the fully connected layer for classifying the input. They present the output from a CNN as a probability that the input image falls into each target class.

Lastly, in a classification task, a softmax layer will provide the probabilities of the image being of each class, usually the class with the highest probability is chosen as the output label.

CNNs have notably transformed the field of visual recognition. They are fundamental to applications such as video/scene analysis, self-driving cars, object detection, and many more. While CNNs excel in image-related tasks, they’ve also shown promise in natural language processing and time series analyses.

### Related terms

Deep Learning, Artificial Neural Networks, Filter, Feature Maps, Pooling, ReLU, Fully Connected Layer (Hidden Layer),, Image Classification, Object Detection, Softmax Function, Natural Language Processing (NLP),.