What are the basic rules to write a trigger

A pice of apex logic is executed based on DML operations like, 

Before Insert 

After Insert

Before Update 

After Update

Before Delete

After Delete

Before Undelete

After Undelete

use trigger.new for latest data as list of sObjects

trigger.old for previous data as list of sObjects

trigger.newMap for latest data as map of Id and data

trigger.oldMap for old data as map of id and data