VOIP = Voice over IP. From there, it's pretty self explanatory. Pretty much, you use a service (and there are many) to make a phone call via your computer, almost like instant messageing with voice. The program I use is SkyPE and have found it to be all that I need. You can do free "calls" to other members of SkyPE
, provided they're logged on.
You can buy a phone # so that people can call you (which probably wouldn't really be beneficial if you'll be traveling, except for voicemail). You can call regular phones from SkyPE(some calls are free, some you have to pay) if you have speakers and a microphone (or a cheap VOIP phone wih a USB port which can now be purchased nearly anywhere). The phone service is still being improved and I've found it "satisfactory" but not superior.
As for traveling - I wouldn't really really on using VOIP unless you'll have your laptop. If you end up at an intnet cafe or something without the program, you have to download and that'll eventually turn into a real pain. Prepaid phone cards are excelling though.