Reminder feature is an extension to standard MOSS 2007 / WSS v.30 feature to enable event email reminder to the users. It also has some other functionality to change the security level of per scheduled item.
The feature consists of two parts, 1) the timer job part and 2) the event receiver part. Timer job is responsible to check scheduled item at specified time called ETS (ETS = Estimated Time to Send), and then execute send mail task and records the execution time in ATS (Actual Time to Send). Event receiver part is responsible to change the security settings of scheduled item when authors create it. Event receiver is also responsible to create reminder information list, which will be referenced by timer job.