SoftPhones allow you to use your computer to make voice calls over the Internet.
SoftPhones are really just software that runs on your computer and emulates a real telephone. They usually render an image of a telephone or a windows interface that provides the user with buttons for dialing, disconnecting, volume controls, etc.
The SoftPhone often has many optional features such as phone book (contact list), redial, call record, conferencing, call history, and perhaps many other optional features.
The future trend is for soft-phones to be web browser based, in which case there is no software to down load.
Generally speaking, there are two categories of soft-phones:
Examples of pre-configured softphones are Google Talk, Gmail's Voice and Video Chat plugin, and Skype.
A good example of a user-configurable softphone is the 3CXPhone softphone. In a later article, I explain how to configure the 3CXPhone with one of my favorite VoIP service providers - CallCentric.