Stem is the part of the word that never changes even when morphologically inflected, whilst a lemma is the base form of the word. For example, from "produced", the lemma is "produce", but the stem is "produc-." This is because there are words such as production.[1] In linguistic analysis, the stem is defined more generally as the analyzed base form from which all inflected forms can be formed. When phonology is taken into account, the definition of the unchangeable part of the word is not useful, as can be seen in the phonological forms of the words in the preceding example: "produced" /pr??dju?st/ vs. "production" /pr??d?k??n/.
Some lexemes have several stems but one lemma. For instance "to go" (the lemma) has the stems "go" and "went". (The past tense is based on a different verb, "to wend". The "-t" suffix may be considered as equivalent to "-ed".)
Difference between stem and lemma
原文：http://www.cnblogs.com/robert-dlut/p/4132163.html