A service instance is a combination of an SRV record containing the service hosts name and port and a TXT record containing meta-information (key=value pairs). Services instances are found via PTR records.
For HTTP services in a browse domain called example.com, you'd query for _http._tcp.example.com PTR records and get back a list of instance names such as HN._http._tcp.example.com. If you then wanted to connect to the "HN" HTTP service you'd then lookup the SRV and TXT records at HN._http._tcp.example.com.
For anyone interested, dns-sd.org hosts some example services — try `dig -t ptr _http._tcp.dns-sd.org` and go from there.