hai santhu,

  Workflow:-Work flow triggers when an DML events like Insert,Upadte Occurs.we canot fire workflows after record has been deleted.

        Workflow actions are     Field Update,Email alert,Task alert and outbound message........ so u can any of action for workflow.so when ever work flow fires that action takes place.

Approval Process:-

 suppose u can think like this, in an Software company there ia an HR team,when ever they opens a postion ,that position has to approved by HR team..if Hr team approved that position then that position is approved and they start open recuritment. if they rejected  position then they doesnot open recuritment.........for that position

 when ever an record submitted to Approval process,the record is going into locking state.if it is approved then it is in locking state forever.if it is rejected then that records comes in to unlocking state.

in approval process we can trigger work flow when   initial sumission,aproval action,rejection action,final rejection.................

Trigger:-  Trigger is also same ,when an Dml event occurs like insert,update,Delete   trigger will fire......

 here in deletion Scenario also trigger will fire....but in work flows workflow cannot fire..........for deletion