University of California, Berkeley: Zvezdelina Stankova-Frenkel's "Unique and Nonunique Factorization"
Read this page. In particular, focus on the exercise in the reading. Do not be intimidated by the notation in the essay, just read it down to the part on ideals.
Click http://web.archive.org/web/20160620180638/http://mathcircle.berkeley.edu/BMC3/bamc/node2.html link to open resource.