The suffix 'ee' is used when a person receives an action, or better put, "person to which something is done". Words such as addressee, consignee, employee and appointee are examples. The addressee no longer lives here. The suffix 'er' labels the person who who has done something - the doer of an action if you will. Words such as teacher, runner, player are examples. Note that simply because we have some words in which a word can take on both suffixes which complement each other such as employer/employee or addresser/addressee etc., it doesn't always hold true with other words that take on the 'er' suffix. For example, you would NOT say the following: The teacher teaches the teachees.

Karol, sorry

Thank you, Carol!

You mean like in "employee" and "employer"? Employee is the one who's employed (~worker) and employer is the one who employs (~boss). Another example that comes to my mind is "trainee - trainer".