"Sons of Liberty" was the name given to a number of protest groups that formed to protest the Stamp Act - and stayed together afterward, organizing Anti-British rallies and enforcing boycotts in the lead up to the revolution. It was like a friendly club for angry men.
Although the groups would later become more coordinated, they started separately. 'Sons of Liberty' was the usual choice for a name because it was a popular phrase meaning "Colonist" at the time. As you might predict given the name, the Sons were mostly middle-class and yes, male - merchants, lawyers, and politicians. Paul Revere was likely a member of the Boston group, to give you an example.
The Sons (in all of the colonies) weren't above using violence and intimidation to get their way - who was? they threatened all of the Stamp Distributors into resigning - with the Boston group going so far as to destroy the local distributor's offices, and parade him through town before he quit in a very public - and likely terrified - resignation speech.
Given that, it probably won't surprise you to learn they earned a reputation for being the radical wing of the Patriots - rallying the people and sometimes losing control of them. A public protest in Boston turned into an angry mob that burned the governor's mansion, some groups of the Sons were also known to tar and feather British officials and sometime Loyalists.
You know, standard good guy stuff. What a really terrific way to blow off steam.