now()
The now() mehtod in Salesforce Apex returns the current Datetime based on a GMT calendar. So, system.now() and Datetime.now() returns Datetime based on a GMT calendar.
now().format()
The now().format() method in Salesforce Apex returns the current Datetime based on the user timezone settings selected in the user detail.
Sample code:
system.debug('System DateTime is ' + DateTime.now());
system.debug('System DateTime is ' + System.now());
system.debug('User DateTime is ' + DateTime.now().format());
system.debug('User DateTime is ' + System.now().format());
Output: