Difference between Before Trigger and After Trigger

Do you wonder always what exactly is the difference between before and after triggers. Here is the small example which gives you better understanding with hello world example.

Before Insert example

trigger HelloWorldTrigger on Account (before insert) {
for(Account a : Trigger.New)
a.Hello__c = 'World';
}

For the same example if you just change the event to after insert then you will end up having an error as record has been locked. The work around for this is to allocate memory and update the same record so that you can avoid the record locking issue

After Insert example

trigger HelloWorldTrigger on Account (after insert) {
    List<Account> accList = new List<Account>();
    for(Account a : Trigger.New){
        Account acc = new Account(id = a.id, Hello__c = 'World');
        accList.add(acc);
    }
    if(accList.size()>0)
    update accList;
}

Note: Here you have to allocate a memory and then update that record explicitly.

Conclusion:

If you would like to update the same record then you need to use before trigger however if you would like to work with system generated fields then you have to use the after trigger.

Here is the standard Salesforce.com order of execution

Old record loaded from database (or initialized for new inserts)

New record values overwrite old values

System Validation Rules

All Apex “before” triggers

Custom Validation Rules

Record saved to database (but not committed)

Record reloaded from database

All Apex “after” triggers

Assignment rules

Auto-response rules

Workflow rules

Escalation rules

Parent Rollup Summary Formula value updated (if present)

Database commit

Post-commit logic (sending email)