Validation set can be considered as a part of the training set as it is used for parameter selection and to avoid Overfitting of the model being built. On the other hand, test set is used for testing or evaluating the performance of a trained machine leaning model. In simple terms ,the differences can be summarized as- Training Set is to fit the parameters i.e. weights. Test Set is to assess the performance of the model i.e. evaluating the predictive power and generalization. Validation set is to tune the parameters.