A scheduled scan is the one which I scheduled either from SEPM or SEP client to run at a particular time.This scan runs on System Priviledge.
A user need not be logged in for this scan to run.Scheduled scan will run only at a time when it is confugured to run.
Trigger scan is one in which the user manually runs a scan like custom scan, active scan or full scan.The user has to be logged in to run this scan.
this scan runs in logged in users priviledge.A user can anytime trigger a scan