ST01 - System Trace which will be required when you want to record trace for system
ST05 - Performance Trace (Developer Trace) which is required when any userComplaining for a long transaction and you want to record all activity of Transaction.
SM21 - System log is for the recent logs for your CI and application server. What ever happen in SAP it will write a system log.
System Trace: If you want to record the internal SAP system activities, use the function SAP System Trace. Recording the processes in your application server enables you to monitor the system and facilitates troubleshooting.
Developer Trace: Developer traces contain technical information for use in the event of problems with your system. Using the entries in the developer traces requires a sophisticated knowledge of the host systems in which your SAP system is running and of the SAP system itself.
System Log: You can use the log to pinpoint and correct errors occurring in your system and its environment.To work with system logs, choose Tools -> Administration from the R/3 initial screen. When the System Administration initial screen appears, choose Monitoring -> System log. Alternatively, you can choose Test -> System log from the ABAP/4 Development Workbench initial screen.