The Part-of-Speech (POS) tagger marks words with a POS-tag, based on its context and definition. So, for example, the word ‘answer’ is tagged as a Noun or as a Verb, depending on its context.
There are many variants of POS-tag schemes and their abbreviations. In some schemes (e.g. in the Penn Treebank below) the POS tag includes some morphological information. This (partly) depends on the morphological richness of a language. There is also a Universal Scheme for POS tags.
POS-tags are (or were) useful for building lemmatizers and NER systems, but also for information retrieval with rule-based token-patterns. For example, see this Spacy demo for rulebased search for token patterns.
This article is part of the project Periodic Table of NLP Tasks. Click to read more about the making of the Periodic Table and the project to systemize NLP tasks.