You will immerse yourself in a new community and culture, not simply as a tourist, but as a part of an international team. If you are an adventurous traveler and enjoy hands-on learning, don't mind getting dirty, like to meet people from other countries and get your energy from working towards a goal as part of a team, you will gain more than you give, no matter how hard you try.
Projects vary widely from one country to the next.
You could be protecting sea turtles along the coast of Mexico or building a playground for inner-city kids in Wales. You could be developing hiking trails in the countryside of the German Black Forest or restoring medinas in Morocco. You may be the only American volunteer in your group, but this will inevitably accelerate cultural awareness. You will be responsible for making your own independent travel arrangements to your project site. On the first day, you will meet other volunteers and the group coordinator.