Most people are familiar w/ NetMeeting and other software that let you talk to others over the internet. VOIP is similar. However, it also uses your phone - the interface we're all familiar with.
Instead of connecting your phone to the wall, you connect it to a box either the VOIP company provides or you buy on your own. This is the modem. You need highspeed/broadband internet for this. Your modem plugs into your cable/dsl modem and now it can 'talk' btwn your phone and the internet.
Your phone lines already do pretty much what VOIP does, just w/ different/older technology (well, some of it's new w/ optics and such, but...).
How hard is it to setup?
It's actually very easy. Assuming you have the company send you a modem or you buy a kit at the store, you simply plug the modem btwn your current cable/dsl modem and your computer (e.g., the out from your cable modem now goes to the new box and the out from that to your computer). It also has a phone jack for your regular phone.
Once it is on, it will boot up and configure itself.
What are the benefits?
There are many benefits depending on the company you go w/, but here are a few I have (all of these are free w/ the monthly fee- $14.99 for my setup):
Features like caller id, call waiting, call waiting caller id, call forwarding, etc.
Email alerts for new voicemail.
Web access to voicemail - you can play your messages on your computer.
Auto-forward - if the phone is ever down or isn't answered, it's auto-routed to another number
Online management of all features, settings, account, etc.
Computer dialing - highlight any number on your computer, hit F6 and it sets up the call from your phone to theirs - very convinient!
Keep your regular phone (you don't need a voip phone)
There are some other cool features available for extra $ like the softphone which lets you use your computer as a phone w/o the modem - this means you can take your laptop w/ you wherever and you have your phone w/ the same phone number everywhere too (you can do this w/o the softphone, but you have to take your VOIP modem around w/ you - not bad if you're going somewhere for a longer trip).
Another really cool thing is the virtual phone numbers. You can get extra numbers that ring the same phone, but they are local numbers to any area code you want. So if I live in Dallas and have family in New York, I can get a local phone number here and there. That way my family in New York can call a local number (free) and talk to me! Vonage charges $4.99/month for that. I'm not sure about the others.
And the biggest benefit - IT'S CHEAP! I pay $14.99 plus some tax and such, but not the ump-teen taxes, charges, fees, etc. like the phone company charges.
Also, if you do a little research/reading, you can set it up to where every phone jack in your house is wired for VOIP. It's little more than plugging the output of your VOIP modem into the wall jack you already have. Very little more.
You get lots of good, free features and options for doing/managing a lot more including a lot over the web and it's SO much cheaper (at least it is for me!). Being the 'average guy' I am, I don't really use all the features. Fortunately, it's pretty much a 'set it up and forget about it' type thing except that I save $40+/month w/ it!
I use Vonage b/c that's what I was told about first and it was one of the first. There are others and I'm sure they have their strengths, but Vonage has the cheapest base option at the time I did my research.