Natural Language Processing has emerged as the most popular field in Data Science. It deals with the development of statistical models that help computers understand human language.
These statistical models are part of Machine Learning and through several of its algorithms, are able to assist computers in understanding natural language. Python language comes with a collection of libraries called Natural Language Toolkit (NLTK) developed for this particular purpose only.

NLTK is widely used for various language processing techniques like tokenization, stemming, tagging, parsing and machine learning. It consists of over 100 corpora which are a collection of data for building machine learning models.
It has a variety of applications such as Parts of Speech Tagging, Word Segmentation, Machine Translation, Text to Speech Speech Recognition, etc.