Activism is any attempt to intervene (promote, impede, direct, alter, etc.) in social, political or economic or environmental activity or reforms with the intention of improving society. This can be done through "demonstrations", letter-writing or phone campaigns, etc. that are used to raise attention to an issue/issues.
Modern examples of this are the campaigns to Save the Whales, Elephants, Tigers, etc. as well as meeting and demonstrations to support political candidates.