Net telephony tools allow making phone calls from your PC to other users using the same software.
Some Net phone tools also support placing phone calls to traditional land lines and cell phones too, but almost always with a cost. Furthermore, some of the most sophisticated clients can also get a "real" phone number (feature usually has a monthly cost) that people using traditional phones can use to call to you. For most of the Net telephone / VoIP clients, it is somewhat difficult to decide whether they're primarily VoIP tools or Instant Messengers. Which is exactly the reason why you can find many of these tools listed in both categories.