What are the differences between system mode and user mode in Salesforce?

Hello Ajit,

System Mode is nothing but running apex code by ignoring user's permissions. In system mode, Apex code has access to all objects and fields— object permissions, field-level security, sharing rules aren't applied for the current user. This is to ensure that code won’t fail to run because of hidden fields or objects for a user. All apex code run in system mode. It ignores user's permissions. Only exception is anonymous blocks like developer console and standard controllers.

User Mode is nothing but running apex code by respecting user's permissions and sharing of records. Only standard controllers and anonymous blocks like developer console run in user mode.

Apex Triggers always run in System Mode irrespective of sharing settings, is a myth. Because if your apex trigger is calling a class declared using “WITH SHARING”, then all sharing rules for the context user will apply during trigger execution. If you apex triggers is expecting to query all records and that code is running under WITH SHARING context, then user’s profile will come into play.