The difference between Forward and Reverse DNS
A forward lookup zone will, when supplied with a domain name (for example google.com), return the IP address for the supplied domain. When you access Google, a DNS server is looking in its forward lookup zone for the IP address(es) of the site, and then returning them to you. This type of record is an A record, and is the most common. A forward lookup zone can contain other records, such as MX, C-Name etc.
A reverse lookup zone is used to lookup the domain name when supplied with an IP address. You normally supply the IP address backwards and append .in-addr.arpa (more info at http://en.wikipedia.org/wiki/.arpa).
A good tool for finding RDNS or normal DNS records is dig.