Answer:   We can call the apex code by creating object for the class (or) if the variables or methods are static then we can call with class name.

Apex Code in the trigger will execute automatically for the DML operations.

If you want to execute apex code on a specific time then we should write batch class.

With Batch Apex we can process maximum of 50 million records.

Batch Apex is asynchronous (execute in future context).

While writing the batch class we should inherit Database.Batchable interface.

Database is a built in global class which contains inner global interface.