DNS servers are based on a recursive look up as you can see below.

Below you can see XP forwarding the request to its DNS server. The DNS Server then recursively resolves the DNS request. Once it has the answer to the Clients request, it forwards the answer back to the client.

A great tool for DNS troubleshooting is nslookup.

How do I find the root servers ?

C:\Documents and Settings\admin>nslookup
Default Server:  dns.service.isp.net
Address:  194.168.1.100

> set type=ns
> .
Server:  cache2.service.virginmedia.net
Address:  194.168.8.100

Non-authoritative answer:
(root)  nameserver = M.ROOT-SERVERS.NET
(root)  nameserver = A.ROOT-SERVERS.NET
(root)  nameserver = B.ROOT-SERVERS.NET
(root)  nameserver = C.ROOT-SERVERS.NET
(root)  nameserver = D.ROOT-SERVERS.NET
(root)  nameserver = E.ROOT-SERVERS.NET
(root)  nameserver = F.ROOT-SERVERS.NET
(root)  nameserver = G.ROOT-SERVERS.NET
(root)  nameserver = H.ROOT-SERVERS.NET
(root)  nameserver = I.ROOT-SERVERS.NET
(root)  nameserver = J.ROOT-SERVERS.NET
(root)  nameserver = K.ROOT-SERVERS.NET
(root)  nameserver = L.ROOT-SERVERS.NET

How do I find the TLD servers ?

> com
Server:  cache.service.isp.net
Address:  194.168.1.100

Non-authoritative answer:
com     nameserver = g.gtld-servers.net
com     nameserver = d.gtld-servers.net
com     nameserver = j.gtld-servers.net
com     nameserver = h.gtld-servers.net
com     nameserver = c.gtld-servers.net
com     nameserver = i.gtld-servers.net
com     nameserver = m.gtld-servers.net
com     nameserver = k.gtld-servers.net
com     nameserver = b.gtld-servers.net
com     nameserver = l.gtld-servers.net
com     nameserver = a.gtld-servers.net
com     nameserver = e.gtld-servers.net
com     nameserver = f.gtld-servers.net

How do I find zone information ?

> set type=soa
> bbc.com
Server:  cache.service.isp.net
Address:  194.168.1.100

Non-authoritative answer:
bbc.com
primary name server = ns0a.rbsov.bbc.co.uk
responsible mail addr = bofh.bbc.co.uk
serial  = 1225383591
refresh = 86400 (1 day)
retry   = 86400 (1 day)
expire  = 86400 (1 day)
default TTL = 0 (0 secs)