Apex can be invoked through the use of triggers. A trigger is Apex code that executes before or after the following types of operations:
insert
update
delete
merge
upsert
undelete
For example, you can have a trigger run before an object's records are inserted into the database, after records have been deleted, or even after a record is restored from the Recycle Bin.

You can define triggers for top-level standard objects that support triggers, such as a Contact or an Account, some standard child objects, such as a CaseComment, and custom objects.
For case comments, from Setup, click Customize | Cases | Case Comments | Triggers.
For email messages, from Setup, click Customize | Cases | Email Messages | Triggers.
Triggers can be divided into two types:
Before triggers can be used to update or validate record values before they are saved to the database.
After triggers can be used to access field values that are set by the database (such as a record's Id or lastUpdated field), and to affect changes in other records, such as logging into an audit table or firing asynchronous events with a queue.
Triggers can also modify other records of the same type as the records that initially fired the trigger. For example, if a trigger fires after an update of contact A, the trigger can also modify contacts B, C, and D. Because triggers can cause other records to change, and because these changes can, in turn, fire more triggers, the Apex runtime engine considers all such operations a single unit of work and sets limits on the number of operations that can be performed to prevent infinite recursion.


Additionally, if you update or delete a record in its before trigger, or delete a record in its after trigger, you will receive a runtime error. This includes both direct and indirect operations. For example, if you update account A, and the before update trigger of account A inserts contact B, and the after insert trigger of contact B queries for account A and updates it using the DML update statement or database method, then you are indirectly updating account A in its before trigger, and you will receive a runtime error.