Here's the link to the American Dahlia Society's (ADS) home page. The ADS sets the rules for dahlia competition in the US. Not interested in dahlia competition? This is still a great site for lots of dahlia information, including links to other state societies, judging, trial gardens, bench evaluation, etc. Be sure and check out the Classification Guide. By exploring the guide you can find dahlias by name, class, color, form, originator, size, the "Fab Fifty", and/or by various medal winners.
 

Classification Guide
 
Classes

Check out this grid to understand how the various classes are organized. Note, for some of the forms, "form" and "size" are combined.
Color

15 official colors are recognized, but scroll down on this page and you'll see there are lots more. This ADS page says "... the ADS Official Color Guide is the Royal Horticultural Color Chart."
Form Note there are 20 forms listed, but some say there are "19" because the Strait Cactus (SC) and Incurved Cactus (IC) forms compete against each other at the class level.
Originator The originator of the particular variety. One of the fun things to do with dahlias is to grow dahlias from seeds to get new varieties.
Size Nine sizes are listed, but again note, for some of the forms, "form" and "size" are combined ... so check out the grid at "Classes" to understand how form and size interplay.