A neural network is a machine learning algorithm that receives as input a number of variables or features, and finds the best combination of those features to predict an output. The output can be a simple categorisation (e.g. does this event contain a Higgs boson?) or predicting a quantity (e.g. what was the momentum of the particle that left these energy deposits in the detector). A deep neural network is a type of neural network that uses multiple layers to progressively compute more complex features from the raw input in order to achieve a more accurate output.