MNIST is a data set used typically used for «hello world» examples of machine learning.
The data set consists of (70000? = 60000 training + 10000 test? samples) gray-scale images of hand-drawn digits (0 … 9). The size of the images is 28 pixel by 28 pixels, the value of a pixel an integer between 0 and 255(?).
Download the data
The MNIST data can be downloaded from various sources, for example
Fashion MNIST is data set of Zalando's article images that also consists of 60000 training and 10000 test samples, each as well 28x28 pixel graysacle images.