Rosin is a cannabis extract that is produced using heat and pressure instead of solvents. It varies in texture from soft taffy to a thick sticky liquid; and in colour from amber, to brown, to slightly green dark brown.
Rosin is sought after by many THC extract connoisseurs for its honest full-plant-extract flavour that really speaks of the material it was made from. Since the material doesn't come in contact with solvents, rosin is by definition very clean and pure. The heat however does affect the taste profile giving rosin a slightly darker flavour.
Marijuana flowers, or trim pieces, are pressed in a heated press which causes the trichomes to "melt" off the plant material. Rosin offers a solvent-free full spectrum flavour that is truer to the taste of the full plant than other, more refined extracts. Because it's a simple and safe process, rosin can be made at home using easily accessible tools.
Rosin is usually consumed by vaporizing, dabbing, or smoking. Low temperature dabbing or vaporizing generally offers the best taste experience.
A rosin's quality is directly related to the quality of the plant product used, and the temperature / time / pressure at which it was extracted.