Whats voluntary work?

Anything that you do out of your own free will, where nobody told you to do it, can be considered voluntary. Voluntary work is when you do a job for reasons other than profit or to gain monetary income.
In the contemporary context, volunteering means to go somewhere, often to a different country, where you work for a period of time on some project, hosted by an organization that you heard about in your home country. Instead of expecting money for the work you put in, you expect to help out with the project that you signed up with, and you hope to have a great time doing it, meet new people and have jolly experiences. Perhaps you are also hoping to gain new skills that you might use later in your life or back at home. Volunteering can however also be done close to your home. You might volunteer with a non-profit organization on two or three afternoons or evenings a week after your regular job, to give your life a bit of diversity, to give yourself the feeling that you are doing something good in the world.
Note that voluntary does not mean that you cant receive any kind of compensation: Volunteers are often given room and board and sometimes even pocket money. Yet this is not always the case. Some volunteer programs are set up where somebody coming from a richer to a poorer country will themselves contribute a certain amount of money to help support the organizations work, in addition to their own sweat.
Therefore one thing to keep in mind is that there are many opportunities out there and that there are almost as many different kinds of organizations, each ones with their own specialty. It is up to you to discover all of these opportunities and to find out what is good for you.