Apex Trigger :
  A trigger is Apex code that executes before or after the following types of 
operations on database objects:
 insert, update, delete, merge, upsert, and undelete. 
These are similar to database triggers written in Oracle's PL/SQL or Microsoft's tSQL.
 One important note is that all Apex triggers are bulk triggers. 
That means that a trigger can be invoked by a single record transaction or by a bulk 
operation such as an API call that has a batch of up to 200 records.
 For example, if I create an Account in the user interface,
 any triggers on the Account object will be invoked with a single record as the input. 
Conversely,
 if there is an integration sending a batch of 200 records to the API, 
that same Apex trigger on the Account object will be invoked only ONCE,
 but with 200 Account records as input.