Trigger Context Variables - Apex Step By Step

Trigger Context Variables chart for better understanding of their use.

Trigger Context Variables Matrix (Trigger.)	isInsert		isUpdate		isDelete		isUndelete
Trigger Context Variables Matrix (Trigger.)	isBefore	isAfter	isBefore	isAfter	isBefore	isAfter	isAfter
new	true	true	true	true	false	false	true
old	false	false	true	true	true	true	false
newMap	false	true	true	true	false	false	true
oldMap	false	false	true	true	true	true	false

true: represents that we can use variable in the trigger

false: represents that the context variable can not be used.

For eg: for Before Insert, we can not use Trigger.old