What are the extensional and intensional ways to define a set?

"There are two ways to define a set: extensionally and intensionally. I’m not saying there are two kinds of sets: rather, there are simply two ways to specify a set. To define a set extensionally is to list its actual members. That’s what we did when we said P = {Dad, Mom}. In this case, we’re not giving any “meaning” to the set; we’re just mechanically spelling out what’s in it. The elements Dad and Mom are called the extension of the set P. The other way to specify a set is intensionally, which means to describe its meaning. Another way to think of this is specifying a rule by which it can be determined whether or not a given element is in the set. If I say “Let P be the set of all parents,” I am defining P intensionally. I haven’t explicitly said which specific elements of the set are in P. I’ve just given the meaning of the set, from which you can figure out the extension. We call “parent-ness” the intension of P. Spelling nit: “intensionally” has an ‘s’ in it. “Intentionally,” meaning “deliberately,” is a completely different word."