One is singular; the other is plural.
Phenomenon comes to English from Greek through Latin—kind of like criterion, which came directly from Greek.
Singular
Phenomenon
Criterion
Plural
Phenomena
Criteria
Quick and Dirty Tip:
Remember that phenomenon is singular by noting that the non at the end sounds like one.
(Please retain the reference in reprint: http://www.letpub.com/index.php?page=author_education_phenomenon_or_phenomena)