Someone who's employed has a job or is busy with something. Some people believe that employed teenagers get themselves in less trouble than those without jobs.
If you're working, you're employed. Most employed people work traditional full-time jobs, although an increasing number of them are freelancers or have part-time positions. Another way to use this adjective is to mean "used," as when you say, "The employed methods for keeping the classroom calm seem a little extreme to me." The adjective employed comes from the related verb, employ, "use, apply, occupy, or hire."