Excellent introductory video from Google’s Martin Gorner covering the handwritten 0-9 digits recognition problem using the MNIST data set.  Code is Python using the Tensorflow library.

He begins with a single layer network, progresses into a multi-layer network and ends with a convolutional neural network, showing how improvements in techniques correspond to better test accuracy.

Covers softmax, sigmoid, ReLU, matrix transformation, convolutional networks, over-fitting, test and training accuracy.

Well worth a watch for beginners and has better explanations than some full courses online.

Note: they figure out the microphone feedback situation around minute 15 or 16.