The dataset is called MNIST and refers to handwritten digit recognition. You can find more about it on Yann LeCun's website (Director of AI Research, Facebook). He is one of the pioneers of what we've been talking about and of more complex approaches that are widely used today, such as covolutional networks. The dataset provides 28x28 images of handwritten digits (1 per image) and the goal is to write an algorithm that detects which digit is written. Since there are only 10 digits, this is a classification problem with 10 classes. In order to exemplify what we've talked about in this section, we will build a network with 2 hidden layers between inputs and outputs.