All triggers define implicit variables that allow developers to access runtime context. These variables are contained in the System.Trigger class:
Variable and the Usage:
isExecuting Returns true if the current context for the Apex code is a trigger, not a Visualforce page, a Web service, or an executeanonymous() API call.
isInsert Returns true if this trigger was fired due to an insert operation, from the Salesforce user interface, Apex, or the API.
isUpdate Returns true if this trigger was fired due to an update operation, from the Salesforce user interface, Apex, or the API.
isDelete Returns true if this trigger was fired due to a delete operation, from the Salesforce user interface, Apex, or the API.
isBefore Returns true if this trigger was fired before any record was saved.
isAfter Returns true if this trigger was fired after all records were saved.
isUndelete Returns true if this trigger was fired after a record is recovered from the Recycle Bin (that is, after an undelete operation from the Salesforce user interface, Apex, or the API.)
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.
newMap A map of IDs to the new versions of the sObject records.
Note that this map is only available in before update, after insert, and after update triggers.
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.
oldMap A map of IDs to the old versions of the sObject records.
Note that this map is only available in update and delete triggers.
size The total number of records in a trigger invocation, both old and new.
Note
If any record that fires a trigger includes an invalid field value (for example, a formula that divides by zero), that value is set to null in the new, newMap, old, and oldMap trigger context variables.