Nondisjunction is when the chromosomes don't split during anaphase. This causes one cell to receive an extra chromosome and one cell to be missing a chromosome.

Trisomy- an extra chormosome
Fig_5,_Edwards'_1960_Lancet_paper_-_incorrectly_depicts_trisomy_17.png
http://en.wikipedia.org/wiki/File:Fig_5,_Edwards%27_1960_Lancet_paper_-_incorrectly_depicts_trisomy_17.png

Monosomy- a missing chromosome
chrome2.gif
http://www.gurfein.com/cubby/ringchromosome.cfm