VOIP is an acronym for Voice Over Internet Protocol, or in more common terms, phone service over the Internet.
If you have a Broadband Internet connection you can get phone service delivered through your Broadband connection instead of from your local phone company.

Voice over IP allows the home telephone user to save on long distance and local telephone service. Users do not need computers in order to place phone calls using VOIP. Here are some of the ways which VoIP may be used:

VOIP makes it easier to save money on many phone services. With VOIP you can have your choice from a wide selection of area codes -- you may choose to have an area code for Florida while you live in New York. This lets your friends, family, and associates in the area code from Florida to call you without accruing Long Distance charges. If you require a secondary phone line, VOIP allows you to attain a secondary line at an inexpensive rate with the same features as your primary line.