Bayesian methods are a sub-field of statistics (bayesian statistics), and are often used in machine learning. Bayesian methods assume a prior over the space of models (the prior belief), and after observing the data, update the belief over the space of models (the posterior belief). The posterior belief can be used to make predictions, etc. (This is in contrast to frequentist methods that are “designed to create procedures with certain frequency gaurantees (consistency, coverage, minimaxity etc)” (Nonparametric Bayesian Methods, Chapter 8)). Bayesian methods often have very good frequentist properties, and often perform well in practice.
There is recent interest in combining bayesian methods with deep learning. For examples, see the Bayesian Deep Learning Workshop at NeurIPS. Bayesian methods such as bayesian optimization are also used in hyperparameter tuning.
Bayesian nonparametric methods are a sub-area of bayesian statistics, and are also commonly used in machine learning. They were very popular in NLP before the deep learning era started in 2014.