If you travel internationally this is a must. Once on your computer you can talk via the Internet to other Skype computers or call traditional phones i.e. landline or cell. Calls to phones are charged at rates determined by the country the call is to. I have spent the last 6 weeks in Khartoum, Sudan my total phone costs for this tour was less than 20 Euro. I paid AT&T over $4500 US for my last 6 week tour in The Sudan.
BTW Skype will work with dial-up, not as well as with hi-speed but it will work. There will be dropped calls and some distortion on many of your calls when using slow connections. Broadband computer to computer is the best, better than either phone to phone or computer to phone.