System Trace (ST01) vs. Developer Trace (ST05) vs. System Log (SM21)
Please explain to me what is the difference between System Trace, Developer Trace, and System Log?
ST01 - System Trace which will be required when you want to record a trace for the system.
System trace will record the application level activities of sap system like dump error, etc.
ST05- Performance Trace (Developer Trace) which is required when any user complaining for a long transaction and you want to record all activity of Transcation.
Developer trace will detect the development related error and it can be used for a developer to correct the errors.
SM21- System log is for the recent logs for your CI and application server. Whatever happens in SAP it will write a system log.
System logs will detect and correct the error os entire sap systems. Useful for analyze the issues when sap system stop and start issues.