Software

Hi,
As I understand it VOIP requires a subscription service to a VOIP provider, to get you from the world of the internet into the telephone system.
A softphone is software on your computer that acts like a phone but still needs to "talk" with your VOIP provider to get to normal telephones. The problem with a softphone is that you can only ever make phone calls through your computer.

Adapters are boxes that connect to your broadband service and allow the use of standard telephones, your computer can be turned off and you can still make calls (provided that your broadband modem and VOIP adapter are turned on).

How Stuff Works is generally a good starting place when you need knowledge:

http://computer.howstuffworks.com/ip-telephony.htm

http://andyabramson.blogs.com/voipwatch/2004/05/how_voip_works.html