Difference between Trigger.new & Trigger.old

Trigger.new : Returns a list of the new versions of the sObject records. Note that this sObject list is only available in insert and update triggers, and the records can only be modified in before triggers.

Trigger.old : Returns a list of the old versions of the sObject records. Note that this sObject list is only available in update and delete triggers.

For more info go through this link:

https://developer.salesforce.com/docs/atlas.en-us.apexcode.meta/apexcode/apex_triggers_context_variables.htm

https://developer.salesforce.com/docs/atlas.en-us.apexcode.meta/apexcode/apex_triggers_context_variables_considerations.htm

Suppose you have a custom object Custom_obj__c

Trigger.new means it is a List<Custom_obj__c>

and

Trigger.newMap means it is a map<Id, Custom_obj__c>

---

In before insert context your Trigger.newMap always be null because in before context records is not submitted to database so Id is not generated that's why in before insert we don't use Trigger.newMap

But in After insert Id is generated so We can use Trigger.newMap

In case of before and After update since id is already generated in Insert event.

So we can use Trigger.newMap in before and After update