To lots of people, using the Net means looking at YouTube videos or buying books from Amazon.com—but both of these things are really about the World Wide Web, not the Internet. The Internet is the worldwide network that links virtually every modern computer on the planet, and it's made up of telephone lines, satellite links, fiber-optic cables, and old-fashioned copper wires. The World Wide Web (all those shopping sites, home videos, and so on that you browse from your computer) is just one of the things that uses the Internet; email is another.
The Internet is designed so that it can send all kinds of information, in all kinds of different ways, between the various computers that it connects together, and without any kind of rewiring or redesign. (Technically, this is called the end-to-end principle.) That's why, in the mid-1990s, some clever technical people were able to figure out how to send telephone calls over the Net, much like any other kind of information. This was the birth of VoIP.
All the information that travels over the Net—from the latest music videos on YouTube to the confirmation email from Amazon that your book is on its way—is sent by a method called packet switching. Something like an email, which might be pages and pages of characters, isn't actually sent as one big chunk: when it leaves your computer, it's broken down into many small pieces called packets, each of which travels independently across the Internet (theoretically by a completely different route from other packets) before being reassembled into a copy of the original email when it arrives at its destination.
It's a bit like sending a book through the post not as a big fat parcel but by putting every single page into a separate envelope, individually addressed and dispatched. It might sound odd to send things this way, but packet switching is actually an extremely quick and efficient way of handling the billions of emails, web pages, and everything else that has to zip back and forth across the Net every single day. (You can read more about how it works in our main article on how the Internet works.) All the computers connected to the Internet understand how to send and receive packets like this; thankfully, they all agree to work in exactly the same way using exactly the same system, which is known as the Internet Protocol or IP. (One of the key parts of the IP that you may be familiar with is that every computer can be "addressed" by quoting a unique number, known as its IP address, which is a bit like the computer equivalent of a telephone number or building address.
voip definition
How, then, do you send a telephone call over the Internet? There are really three separate problems to solve before you can do it: alerting someone that you want to call them, turning your voice into digital sound and sending it over the Net (and receiving replies in the opposite direction), and "interfacing with" (linking in to) the ordinary telephone network, if your call is going to a traditional landline telephone or cellphone (mobile phone). Let's look at each of these in turn
