The Resistance is a loose, covert network of humans and other humanoid species with the shared goal of defeating the Combine and restoring their freedom.
The Resistance has no central command structure, however, there are central figures. Romanov, Asya, Boleslav, and Harry are examples of Resistance leaders. The Resistance is divided into two areas: Combat and Research. Scientists such as Romanov command the research division, whereas leaders such as Asya and Harry tend to command the combat section.
The core Resistance army is divided into four sections: regular resistance troops, who are fully fledged resistance members, equipped with body armor and supplies (some are trained medics as well); the civilian militia forces, who are citizens who have just gained weapons; And the refugees, who are tasked with aiding the escape of civilians and fighting the Civil Protection.
In the Combine controlled world, humanity readily utilizes stolen Combine technology. In order to distinguish converted Combine tech, they will often be sprayed with lambdas or other designs.
Humanity also uses its existing technology well, scavenging and repairing radios, transmitters, receivers, T.V. Screens, cameras, computers, etc. Cars, watercraft, and structures are built by hand. Despite the Combine rule, the Resistance science teams have created weapons like the Gravity Gun, Portal Gun, and a teleporter smaller and more advanced than the Combine's teleporter. The Resistance was an underground movement in City 38, where most actions were covert. Spies infiltrated the Civil Protection by going undercover, in order to assist civilians in escaping City 38, and possibly alerting people of inspections and raids. Outside of the city, however, the Resistance is able to make frequent offenses against the Combine, and provide itself with sufficient defense.
The Resistance generally operates as a shadow movement similar to many terrorists or rebel organizations in the modern world; unable to take on the Combine Empire directly, the Resistance seeks to undermine it by strategic moves. The Resistance held several strong points in and around City 38, before they were destroyed by the Combine. Outside of City 38 is Black Forest, a very well suplied base in a forest.
Possessing no means to mass manufacture weapons, the Resistance doesn't have standard issue weapons like the Combine forces do. They generally scavenge weapons from the Combine, either through killing them or finding them elsewhere. The Resistance makes use of any resources they can find, often building things out of scrapped parts and making specialized equipment, such as the Crossbow and Gravity Gun, and vehicles, such as the Scout Car, the Airboat and the Muscle Car by hand. They are also known to use pre-existing weapons left over from before the Combine invasion. Throughout City 38, the Resistance have sprayed graffiti and posters as a protest to the Combine regime.