What is it called and how well does it work? (See details please)

You can buy one at Office Max for about twenty dollars. It jacks into your computer and somehow you now have phone service. I’ve seen them advertised on tv, and I saw a display of them the last time I was in Office Max. I have a lan line as part of my computer service but I don’t use it. I even have a phone number.
Do any of you know what I’m talking about, and if they work as advertised? The store rep didn’t know.