Trigger is mainly used for the situation when the db automeically trigger some events.
DML Triggers are,
Insert trigger,update trigger,delete trigger
Assume u write the delete trigger for your table.
whenever a row is deleting that table the trigger will raise and the corresponding codes will be executed.
for ex: In your delete trigger u delete the subcategories and products.
assume trigger table consists of categories.
whenever the category is deleting your delete trigger will raise and autometically corresponding subcategories and products will be deleted.
Like that works all trigger events.