It explains about how to delete existing records, how to restore records from recyclebin and how to delete records permanently .
References:
https://developer.salesforce.com/docs/atlas.en-us.apexcode.meta/apexcode/langCon_apex_dml_examples_delete.htm
https://developer.salesforce.com/docs/atlas.en-us.apexref.meta/apexref/apex_methods_system_database.htm
