Client enters a domain name (www.domainname.com) into his browser
The browser contacts the Client's ISP for the IP address of the domain name
The ISP first tries to answer by itself using "cached" data.
If the answer is found it is returned. Since the ISP isn't in charge of the DNS, and is just acting as a "dns relay", the answer is marked "non-authoritative"
If the answer isn't found, or it's too old (past the TTL), then the ISP DNS contacts the nameservers for the domain directly for the answer.
If the nameservers are not known, the ISP's looks for the information at the 'root servers', or 'registry servers'. For com/net/org, these start with a.gtld-servers.net.
NOTE: The 'whois' information is never used for DNS, and is often misleading and inaccurate