Rosin is a concentrate made without any solvents. Material is placed in a fine mesh bag between two heated plates and pressed, and the warm pressure squeezes out the resinous oil, which is collected on parchment. The starting material can be flower, kief (dry sift) or hash.
Because it uses only heat and pressure, rosin is classed as solventless. Many people are drawn to it for that reason and for its full flavor, since pressing at moderate temperatures keeps a lot of the plant's terpenes.
The quality of rosin depends heavily on what goes into the press. Flower rosin is the most basic. Hash rosin, pressed from ice-water or dry-sift hash, is usually cleaner and more flavorful. Live rosin is hash rosin made from fresh-frozen plants.
After pressing, rosin is sometimes "cold cured" or "jam cured" in a jar for a few days or weeks, which changes the texture into a badder, jam or sauce.
Rosin is usually more expensive than solvent-based concentrates because it takes more starting material and labor. Dab it at lower temperatures for the best flavor, start with a small amount, and check the COA for potency and contaminant screening.