Each domain name is constructed from three words, plus the extension.
Each of the three words is chosen randomly from one of three different words lists.
List #1 contains over 7,500 five-letter words.
List #2 contains about 1,000 three-letter words.
List #3 contains 150 adjectives of differing lengths.
Just using those three lists would provide a total possible combinations of 1,125,000,000 domain names. Enabling or disabling 'use hyphens' doubles that number and being able to choose the order that the lists are utilized, the total possible combinations become so great that it staggers the mind.
It's a good idea to select 'Adjectives' for one of the word lists, otherwise all your domain names would be of exact equal length.
*Auto-check Availability is done with a DNS query to this server, not a real whois lookup.