Partisans, guerrillas, insurgents, resistance fighters, and militia are all essentially the same thing. They are soldiers or civilians that become combatants, but not part of a dedicated outfit in a nations armed forces.

Symmetric warfare has soldiers operate in organized groups, with dedicated logistics and supply and support companies, that communicate and work together with other squads, divisions, battalions, etc usually along a defined battle line. This is your typical “army”.

Asymmetrical warfare is designed to hurt the enemy where small groups can do it best. In the example of Soviet partisans, they did not fight the German army directly. They blew up bridges, ambushed convoys, derailed trains, destroyed communications networks, undertook assassinations, committed various forms of sabotage, and executed small scale raids, etc.

Asymmetrical fighters are usually soldiers that have been cut off behind enemy lines, or civilians that take up arms against occupations. They are small and isolated, which makes them difficult to root out and destroy.

Such fighters are meant to operate in secret. They strike and then disappear, either by going into hiding or blending in with the civilian population. Which is why counter partisans usually punished local civilians for partisan action.