Stemming algorithms operate by chopping off the end or beginning of a word while taking into consideration a list of often occurring prefixes and suffixes in inflected words. In some situations, indiscriminate cutting might be successful, but not always.
On the other hand, lemmatization takes into account the morphological examination of the words. To do so, full dictionaries must be available for the algorithm to search through in order to relate the form back to its lemma.