60,000 28x28 grayscale images of hand-written digits 0-9, i.e. 10 categories.
This is THE "OG" computer vision dataset.
Playing with it is the de-facto computer vision hello world.
But it is important to note that as of the 2010's, the benchmark had become too easy for many application.
The dataset can be downloaded from yann.lecun.com/exdb/mnist/:but doing so is kind of pointless as both files use some crazy single-file custom binary format to store all images and labels. OMG!
wget \
http://yann.lecun.com/exdb/mnist/train-images-idx3-ubyte.gz \
http://yann.lecun.com/exdb/mnist/train-labels-idx1-ubyte.gz \
http://yann.lecun.com/exdb/mnist/t10k-images-idx3-ubyte.gz \
http://yann.lecun.com/exdb/mnist/t10k-labels-idx1-ubyte.gz
OK-ish data explorer: knowyourdata-tfds.withgoogle.com/#tab=STATS&dataset=mnist