in addition to starbucks helpful into, links that may help you understand a bit more:
http://www.mirc.com/ircintro.html
http://www.mirc.com/links.html
http://docs.dal.net/docs/netsplits.html

re servers sharing info every so often, i think perhaps he meant that the servers and services bots like nickserv "sync" (synchronize) on a regular basis to communicate/update info to each other, but that doesnt effect chatting.