How do addError() methods work inside triggers, and can they be applied to specific fields?

addError(String msg) marks a record (or a field) as failed with a custom message and blocks that record's save.

• Record level: acc.addError('Cannot save'); shown at top of page.
• Field level: acc.Phone.addError('Phone is invalid'); shown next to the field.
• In delete triggers use it on Trigger.old records.
• In bulk operations with allOrNone=false, only the flagged records fail; with the default, the whole DML rolls back.
• Use addError(msg, false) to disable HTML escaping only when the message is trusted.

It is typically used in before triggers for validations.