What is a training set, validation set, and test set?

Data is split into three subsets. The training set is used to fit the model parameters. The validation set is used during development to tune hyperparameters and compare models — it gives an unbiased estimate of model performance while you still have the ability to adjust. The test set is held out until the very end and used only once to give a final, unbiased evaluation of the chosen model. A common split ratio is 70/15/15 or 80/10/10.