VoIP stands for Voice over Internet Protocol. Put simply, it means using existing data communications networks to carry telephone calls. For residential users, this generally means using the Internet for phone calls which generally results in lower phone bills. VoIP calls can be placed through either standard telephones (a.k.a. "hardphones") or computer software (a.k.a. "softphones").
The term softphone means that a computer is used to place or receive a phone call through the Internet. A well known example of a softphone is Skype. This term is typically opposed to hardphones which are standard phones used to place Internet calls. For example, Vonage is a provider of hardphone VoIP service.