An Apex Trigger is code that executes before or after specific DML events on a Salesforce object — insert, update, delete, undelete. Defined as trigger MyTrigger on Account (before insert, after update) { ... }.
Trigger context variables give you access to the records being processed and metadata about the transaction:
Trigger.new— list of new versions of records (insert/update).Trigger.old— list of previous versions (update/delete/undelete).Trigger.newMap,Trigger.oldMap— same as above keyed by record Id.Trigger.isInsert,Trigger.isUpdate,Trigger.isDelete,Trigger.isUndelete— type of operation.Trigger.isBefore,Trigger.isAfter— timing.Trigger.size— number of records.
Best practice: one trigger per object, calling out to handler classes that decide what to run based on context. Putting business logic directly in the trigger creates maintenance pain.