NLTK is a leading platform for building Python programs to work with human language data. It provides easy-to-use interfaces to over 50 corpora and lexical resources such as WordNet, along with a suite of text processing libraries for classification, tokenization, stemming, tagging, parsing, and semantic reasoning.
via Natural Language Toolkit — NLTK 2.0 documentation.
NLP is a highly interdisciplinary field of study comprising of concepts and ideas from Mathematics, Computer Science and Linguistics. Naturally occurring instances of human language, be it text or speech, are growing at an exponential rate given the popularity of the Web and social media. In addition, people are increasingly becoming more and more reliant on internet services to search, filter, process and, in some cases, even understand the subset of such instances they encounter in their daily lives.
NLP = Natural Language Processing