What exactly is Skype?

I have been offered an online job doing Administrative istant work. Part of my duties would be calling clients back and answering any questions that they may have. I would be provided a Skype account by my employer that I would use to call clients back. My question is, what is Skype? Is there a web cam involved or is it just a voice service? I'm really confused and don't understand what this service is. Please help, I don't want to come off as technologically illiterate, but I just haven't heard much about Skype. Thank you!!!