We often hear these different names, and it can be confusing.
Executors are appointed in the deceased’s Will, and their authority to deal with the administration of the estate stems from the Will. Sometimes they need to apply for a Grant of Probate to confirm their authority to be able to deal with certain types of assets.
If someone has died without a Will, they won’t have appointed an executor. Instead, someone will need to apply to the Probate Registry for a Grant of Letters of Administration, which gives that person(s) the authority to deal with the administration of the estate.  They are then known as administrators.
Those who can apply mirror those who inherit under the rules of intestacy.
Both executors and administrators can be referred to as ‘personal representatives’.