With VoIP, you use an IP network through an ADSL Internet connection or other, to make or receive calls to / from terrestrial PSTN. You can even talk to people with a landline number using your computer, for example using the SkypeIn and SkypeOut service. PSTN includes telephone numbers and IP networks have IP addresses. How these two work together?
Answer: The answer is address translation. Your computer or VoIP device connected to the Internet has an IP address assigned to it. When you call a person on a fixed PSTN phone, your call is forwarded to the PSTN network via the telephone number of the person you call. And vice versa.

Getting the Most out of VoIP!
Here are some things you need to know to be able to make your VoIP experience as great as possible. These include materials and equipment, tips and tricks, security and some other things. VoIP can also handles voice, but also video and data. Here are your resources.
Requirements and Setting Up | VoIP | Equipment/Hardware | Tips And How To's | VoIP Security | Faxing over IP | Recording Your Calls