Apex Trigger Context Variables – Biswajeet Samal's Blog

All triggers define implicit variables that allow developers to access runtime context.
These variables are contained in the System.Trigger class:

Variable	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. This sObject list is only available in `insert`, `update`, and `undelete` triggers, and the records can only be modified in `before` triggers.
`newMap`	A map of IDs to the new versions of the sObject records. This map is only available in `before update`, `after insert`, `after update`, and `after undelete` triggers.
`old`	Returns a list of the old versions of the sObject records. This sObject list is only available in `update` and `delete` triggers.
`oldMap`	A map of IDs to the old versions of the sObject records. This map is only available in `update` and `delete` triggers.
`size`	The total number of records in a trigger invocation, both old and new.