Since VoIP hasn't yet reached full scale adoption, there are many who are curious about the technology and want to use it but aren't sure whether or not they have the correct equipment for it. To clarify things at the start, VoIP is all about flexibility. There are many ways to make a VoIP call unlike the traditional PSTN system and this is VoIP's strength as well as perhaps its weakness. But to use basic VoIP services you need nothing more than an Internet enabled computer and a microphone set. This will allow you to talk to your friends on any service such as Google, Skype, and Yahoo for free.
If you're willing to pay a small amount for more features, you can use Google voice or Skype to make calls to regular phone lines as well. The call rates are cheaper than what you would normally pay and International calling is dirt cheap. If you need mobile communication, then you must get a mobile smartphone which allows you to install applications on it. There are many such phones available on the market today. Any Android phone or iPhone will fit the description. Then, depending on which service you want to use, install that program on the phone (for example, the Skype application) and you're ready to go wherever an Internet connection is available, be it wifi or just a data plan.
Of course, this is for personal usage. If you're a business, things can get slightly more complicated since you'll be needing to connect a lot of people, enable features such as extension dialing, conferencing, GUI configurations, HD voice and all the other paraphanelia that come with a VoIP service. These features are probably the main reason for businesses choosing VoIP even more than the cheaper costs!
Again, there are many ways to go about it. You can buy, install, and maintain all the equipment yourself if you wish, but this is expensive and time consuming. Rather, go for an SIP VoIP provider who will give you access to their VoIP servers and you just have to install any SIP client, give it the IP address of the VoIP servers and it will take care of all the hassle for you. Such a system is extensible, easy to use, and above all affordable. Small and medium sized businesses are turning to VoIP providers in large numbers and if you have a business, it's probably the best solution for you as well.