The meaning of the word partisan has changed over time.
Originally, in the 16th century, it referred to a particular type of pike, having long spear blade as a head with a pair of curved lobes at the base. . Over time it gave its name to the people who continued to use it instead of moving on to more "advanced" lumps of metal such as the halberd. 
These days, the word has come to mean one who takes a certain point of view and promotes it, often with slim regard for the truth or reality. Many partisans have accomplished good things in the world due to their single-minded perspective. However, many have also enabled terrible things.
 In Yugoslavia
See also: Yugoslavia
The Partisans led by Tito also kicked serious Nazi ass in Yugoslavia during World War II, except for one slight problem of them being Communists and spending as much effort fighting the monarchist Chetniks - who were also fighting the Nazis - and preparing terrain for Yugoslavia with revolutionary actions, mostly excecutions, as they did fighting the Nazis. It gets complicated from here but to oversimplify, Stalin had told Tito to stop fighting the Chetniks dammit and get on with the task of building a united anti-fascist front against the Nazis, Tito essentially blew him off and kept fighting the Chetniks, and so three years after the end of WWII the split between Tito and Stalin blew wide open and Yugoslavia wound up becoming a western ally during the Cold War anyhow even though they were still Communist. The result was a notoriously ugly and unreliable car called the Yugo. One year after the Yugo was introduced, the Berlin Wall came down. Coincidence? We think not. What was the topic again?
In the modern era, partisan has come to mean toeing the party line in politics, regardless of the usefulness of the position taken.