Hoople - 3:31 am on Jul 27, 2013 (gmt 0)
The browser passes the DNS request to the operating system. The OS then looks in it's configuration for a DNS setting. It then sends the request to the first server if there is more than one.
The 'finding' of the DNS can come from one of three ways: 1) At startup when the machine asks the 'network' what it's IP is the response contains a DNS setting of one or more servers. 2) Someone manually set the DNS servers in the operating systems configuration. 3) Rarest is no DNS or IP is initially set and the OS then assumes an IP address and 'Broadcast' requests a DNS lookup.
Order to check is an OS behavior. The DNS spec call for moving to the second (and any beyond) only when the first DNS server does not respond AT ALL. Some supposed references state the move occurs when the first server responds with a 'no record' response. Not real bad but definitely different! There are a few OS' out there that do this....
Above is all very high level with no 'fine point' specifics. Ask more specifics and we can give more....I'm not a DNS guru but I have been involved in DNS since before it existed.