A volunteer is someone who serves in a community primarily because they choose to do so. Many serve through a non-profit organization – sometimes referred to as formal volunteering, but a significant number also serve less formally, either individually or as part of a group. Because these informal volunteers are much harder to identify, they may not be included in research and statistics on volunteering.

Beyond personal choice, a number of factors exist around individual motivation and around the rewards for doing service within the community. There is in most cases a range of opinions about the point at which this service changes from volunteering to something else.
Source: Wikipedia

Please read these articles before you consider volunteering:
You're better off backpacking - VSO warns about perils of 'voluntourism'
The Complete Guide To Volunteer Tourism

South Africa has many different volunteering opportunities. Activities range from teaching to wildlife conservation. Below are the organisations that Alternative Route associates with.