It's a task breaking up a text into individual sentences, using cues like periods or exclamation points.

University of Michigan - Ann Arbor

It's the task of determining that two words have the same root, despite their surface differences.

Lemmatization

An on-going but a helpful book resource about NLP
https://web.stanford.edu/~jurafsky/slp3/

Stemming refers to a simpler version of lemmatization in which we mainly just strip suffixes from the end of the word.