Who owns a domain, and what the servers are can be found by querying this root server for information. Keep in mind that most DNS nameservers store the results of past queries for a certain period of time. If the local DNS server doesn't know the IP address for the domain being queried, it will ask the ROOT server for that domain. The ROOT server will point the local server to another server which hosts resolution for the desired domain. The ROOT server can only do this however, if the owner of the target domain has registered a nameserver, and configured that nameserver to perform resolution.
Every client application contains a resolver.
This means that your web browser, or your FTP software, or whatever Internet application you are using tries to resolve any name you type into it, into an IP address. This is called forward resolution. In some cases, an IP address is resolved back into a name. Looking up a name from an IP address is called reverse resolution. The secondary nameserver can be thought of as the slave, or backup.