(also called part-of-speech tag, POS tag or morphological tag) is a label assigned to each token in an annotated corpus to indicate the part of speech and grammatical category. The tool used to annotate a corpus is called a tagger. A collection of tags used in a corpus is called a tagset.
The most frequently used tags in a corpus are listed on the corpus information page with a link to the complete tagset.
Our blog post on POS tags explains how they work.