Hi I have written a trigger on a custom object call Job__c which has a lookup relationship to Account. This trigger is updating a field from Job on the Account record when the Job__c is the first related list. However, I just realized trigger only works for each time a job is either inserted or edited. 

How can I update all the existing Accounts with their jobs all at the same time? The first is a date/time field that is updating from Job to Account. I need to run a report for all the Accounts. Now I need to run a batch apex class. '

But what I don't understand is how the batch apex is run. Does it call it trigger class or trigger calls the batch class? I need the batch to happen only once so that all my existing accounts get updated. Then I can keep my trigger active for all the new job that comes in.

I have read the salesforce doc about batch apex and searched multiple times but I'm still not understanding how it works.
http://www.salesforce.com/us/developer/docs/apexcode/Content/apex_batch_interface.htm

Here's my working trigger: 
trigger NewJobTimeStampOnAccount on Job__c (after insert, after update) {

  Map<ID, Account> parentAcct = new Map<ID, Account>(); //Making it a map instead of list for easier lookup
  List<Id> listIds = new List<Id>();
 
if(trigger.isInsert || trigger.isUpdate){
  for (Job__c childObj : Trigger.new){
    listIds.add(childObj.Account__c);
   
  }
}
  //Populate the map. Also make sure you select the field you want to update, amount
  //The child relationship is more likely called jobs__r (not job__r) but check
  //You only need to select the child jobs if you are going to do something for example checking whether the job in the trigger is the latest
  parentAcct = new Map<Id, Account>([SELECT id, New_Job_Start_Date__c, (SELECT ID, Date_Time_Taken__c FROM Jobs__r) FROM Account WHERE ID IN :listIds]);
 
  List<Job__c> i = [select id from Job__c where Account__c in :listIds order by Date_Time_Taken__c ASC limit 2];


  for (Job__c job: Trigger.new){
       if(i[0].id == job.id && job.Account__c != null)
     {
        Account myParentAcct = parentAcct.get(job.Account__c);
        if(myParentAcct != null)
        myParentAcct.New_Job_Start_Date__c = job.Date_Time_Taken__c;
       
     }
  }
  update parentAcct.values();
 

}

Please help me understand how should I be writing my batch class? How does batch class updated multiple records at the same time or how does it know to update ALL the accounts in my org at the same time?