HuffPost: HumanKind brings you an advent calendar of kindness, celebrating good deeds and the people doing them, in order to inspire and bring hope this festive season. Find out more about the series, and uncover new stories daily here. 

Volunteer work can include any activity that involves spending time, unpaid, doing something that benefits the environment or someone else. So, what drives people to give up their time for others?

We asked volunteers in different fields to explain what their work means to them.

HumanKind is HuffPost’s celebration of kindness, featuring people who do incredible things for others or the planet – transforming lives through small but significant acts. Get involved by joining us on Facebook. You can also nominate those who deserve to be shouted about by filling in this form or emailing natasha.hinde@huffpost.com.