For some jobs, of course, a professional must be licensed. For other jobs, someone who has done one type of work or another for a number of years could be called a professional.

There may be some people who haven't worked at one type of work but who are very skilled at it. I guess they could call themselves "professionals" too.

Electrical and plumbing people have to be licensed. Then there are professional companies (like landscaping companies) who don't hire professionals to do the work. Of course, there's also your basic "lawn guy".