Stemming and lemmatization are very two very popular ideas that are used to reduce the vocabulary size of your corpus.
Stemming usually refers to a crude heuristic process that chops off the ends of words in the hope of achieving this goal correctly most of the time, and often includes the removal of derivational affixes.
Lemmatization usually refers to doing things properly with the use of a vocabulary and morphological analysis of words, normally aiming to remove inflectional endings only and to return the base or dictionary form of a word, which is known as the lemma.
If confronted with the token saw, stemming might return just s, whereas lemmatization would attempt to return either see or saw, depending on ...
