Diction is the art of enunciating with clarity. Proper diction is enunciating words clearly; when singing you should always drop your jaw and use consonant sounds such as: 'eeh', 'eh', 'ah', 'oh', and 'ooo'. When singing, always keep your vowels tall.
For more information on diction, visit this site: http://en.wikipedia.org/wiki/Diction
These pictures were taken by Caitlin.