A root server is used when a computer tries to look up a domain. For example, if a person types "www.google.com" into a web browser, their computer will ask their ISP what the IP address for "www.google.com" is. If the ISP's DNS server does not know (does not have it cached), it will ask a root server which DNS server can tell it the address. The ISP's root server will then get the address from the DNS server that the root server tells it to.