Some services only let you place calls to others using that particular service, while other services function more or less like a traditional phone line, letting you call other phones around the world, and to be called by others outside the network.
Depending on the type of VoIP service you subscribe to, how you place a call differs.
With some services you plug in a microphone headset to the computer and your good to go with VoIP dialing using your keyboard. Other VoIP services will let you plug in your existing phones in to a telephone adaptor that connects to your cable or dsl modem.
VoIP is usually cheaper than a regular phone line, both in fixed costs and call costs. You no longer have to take out a second mortage on the house just to make a phone call half way round the globe.
Is your broadband fast enough for VoIP? Take the Internet Speed test to find out.