VoIP stands for the Voice-over-Internet Protocol. As the name would suggest, it's the protocol used to support voice communication across the Internet and other computer networking. There are a number of private and open source protocols available, which have been used to provide the means of implementing VoIP. There is no standard VoIP protocol.
Some of the most widespread VoIP protocols are: IMS, Skype protocol, MGCP, Skinny, SIP, SAPv2, RTP, RVP over IP and SDP. The advantage of VoIP services is that they often offer free or low cost phone calls. And, are usually more competitive than commercial landline services; from companies such as: BT and TalkTalk.
Alongside the voice call services, online VoIP companies, such as Skype, are also able to provide additional services, such as: video calling, group video calling, text and instant messaging. The merging of voice and data communication infrastructure is a key attraction to VoIP.
Flexibility is another benefit of VoIP. Multiple computers can
access the same ISP account, through the use of routers and other
networking technologies. This means that multiple persons can simultaneously
send 'calls' using the same landline telephone line; something which
is not possible with a copper line telephone service.
As you would expect, there are some disadvantages to VoIP. The principle of which is the quality of the communication. The IP infrastructure of the Internet does not automatically lend itself to seamless vocal communication. Packet loss and delay can result in voice echo's and jitters.
The security of online communications is another issue, with hackers able to access online accounts. However, VoIP supports a secure protocol: named SRTP (Secure Real-time Transport Protocol). This protocol provides encryption of voice communication.