(Of course, similar questions have been asked in the past and there are many sites, papers, video lessons, online that explain how CNNs work, but I think it's still a good idea to have a reference answer that hopefully will give you the main ideas behind CNNs.)

A convolutional neural network (CNN) is a neural network that performs the **convolution** (or cross-correlation) operation typically followed by some downsampling (aka pooling) operations. 

The convolution operation comes from the mathematical equivalent and homonymous operation, which is an operation that takes as inputs two functions $f$ and $h$ and produces another function $g$ as output, which is often denoted as $f \circledast h = g$, where $\circledast$ is the convolution operation (or operator).

In image processing, the convolution is used to process images in multiples ways. For example, it is used to remove noise (e.g. the convolution of a noisy image with a Gaussian kernel produces a smoother image) or to compute derivatives of the image, which can then be used e.g. to detect edges or corners, which are usually the main features of an image. For example, the [_Harris corner detector_][5] makes use of the partial derivatives (in the $x$ and $y$ direction) of the image to find corners (or interest points) in the image.

In the case of CNNs, $f$ is the image (in the case of the first layer of the CNN) or a so-called _feature map_ (in the case of hidden layers), $h$ is the kernel (aka _filter_) and $g$ is also a feature map. (In [this answer][2], I explain these concepts, including how an image can be viewed as a function, more in detail, so I suggest that you read it).

Here's an illustrative example of how the convolution works.

[![enter image description here][1]][1]

where the $\color{blue}{\text{blue}}$ grid is the image, the $\color{gray}{\text{gray}}$ grid is the kernel and $\color{green}{\text{green}}$ grid is the feature map (i.e. the output of the convolution between the image and the kernel). The white squares are around the images represent the padding that is added around the image so that the convolution operation produces a feature map of a specific dimension.

As opposed to many operations in image processing where the kernels are typically fixed, in the context of CNNs, the kernels are **learnable** parameters, i.e. they change depending on the loss function, so they are supposed to represent functions that when convolved with their respective input functions are useful to extract meaningful information (i.e. features) to solve the task the CNN is being trained to solve. For this reason, the convolution is often thought of as an operation that extracts features from images. In fact, the output of the convolution, in the context of CNNs, is often called _feature map_.

There are many different variations of the standard convolution operation. For example, there are **transposed convolutions**, **dilated convolutions**, etc., which are used to solve slightly different problems. For example, the dilated convolution can be used to increase the receptive field of an element of a feature map (which is typically a desirable property for several reasons). There are also **upsampling** operations which are particularly useful in the context of image segmentation. These different convolutions and their arithmetic are explained very well in the paper [A guide to convolution arithmetic for deep learning][3] by Vincent Dumoulin and Francesco Visin. [Here][4]'s also the associated Github repository that contains all the images in this paper that show how the convolution operations work (from which I also took the gif above).

To conclude, CNNs are very useful to process images and extract features from them because they use convolution operations. They can be used for image (or object) classification, object detection (i.e. object localization with a bounding box + object classification), image segmentation (including semantic segmentation and instance segmentation), and possibly many other tasks where you need to learn a function that takes as input images and needs to extract information from those images.


  [1]: https://i.sstatic.net/8fjfx.gif
  [2]: https://ai.stackexchange.com/a/21401/2444
  [3]: https://arxiv.org/pdf/1603.07285.pdf
  [4]: https://github.com/vdumoulin/conv_arithmetic
  [5]: http://www.bmva.org/bmvc/1988/avc-88-023.pdf