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An A and AAAA record are actually primary DNS records. They associate a domain name with a specific IP address, so that when a user types in a web address, such as "www.rackspace.com" their browser knows where to go for the actual website.
The difference between A and AAAA is this: A is IPv4 and AAAA is the current IPv6 record. Beyond that, there is no difference. Later on, IPv6 records will be changed to A6 records.
Great explanation! Also, while it is community driven, the Wikipedia entry for DNS record types is a really helpful resource for reference or just education in general: