Declare Trigger:

DeclareTrigger rules to cause an activity to run when instances of a specific class are created, updated, or deleted in the database. This implements a form of forward chaining.

Database Trigger:

A database trigger is procedural code that is automatically executed in response to certain events on a particular table or view in a database. The trigger is mostly used for maintaining the integrity of the information on the database.

Declare Trigger is specific to PRPC whenever class gets updated, deleted etc it will get fired.

Hi

Though the name trigger is used in both occassions we should not confuse between the two operatios. 

PRPC Declare trigger can detect any specified action ( e.g save or delete ) and fire an activity . Whereas DB trigger ,even though can sense events like save etc but can never fire Pega activities.

Thanks for your replies..

Can Declare Trigger work , if commit for the record  hasn't happened ?

you mean If commit is failed Can Declare Trigger invokes the activity ?