The Data Collector listens to a named TCP-Port, receives the packets with the check results from the agents and write that results to the database error table and to the reporting table.
There are different variants possible for the Data Collector, e.g.
A Data Collector CGI script
A standalone Data Collector daemon
A Data Collector script, which will be called from procmail and works with XML over SMTP :-) Data
The CGI script collectorcgi.pl is the first implementation of a Data Collector, perhaps the mail script will be the second.
Every Data Collector just extracts the real XML data from the agents (that is: from the HTTP stream or from the email message or ...) and gives that XML data to a perl module OpenSMART::collector, which imports that data to the database.