What are VOIP calls…….?

I was reading some product review of apple headphones that came with a microphone and someone said the product is a must for those who want to make voip calls, on the ipod touch. What is that?
So basically, I can call people on my iPod Touch for free?

Voice Over Internet Protocol

Essentially it’s voice communication using high speed internet instead of a regular telephone network

Product Reviews I : Sony MDR-7506 Headphones Product Review