The browser service maintains a list of all resources available on a network. This list includes servers, pcs, printers, shares, etc. When you browse your Network Neighbourhood in explorer, your machine is usually reading that list from the Master Browser.
Like most services in Windows (like DNS, WINS, etc) the Browser service has a Master and any number of backup browser. These backup browsers (as the name implies) hold a backup list of resources on the network. If the Master browser is unavailable, one of the backups is "elected" to be the new master.
Elections take place through broadcast traffic on a network. This can impact network performance, if you repeatedly have browser elections. In NT, the default setting for the browserservice is automatic. It will maintain a browser list, and become a Master browser if necessary.
Elections take place on a strictly hierarchical system. -Basically the highest ranking machine wins.
For more info on browsing check out
http://support.microsoft.com/default.aspx?scid=kb;en-us;Q102878
http://support.microsoft.com/default.aspx?scid=kb;en-us;Q188001
http://support.microsoft.com/default.aspx?scid=kb;en-us;Q188305
(as usual, remove any spaces or %20 that appear in the URL).
Hope this helps,
Gary
