Liquid K2 is one of the many names given to synthetic cannabinoids. It is made by spraying synthetic chemicals onto shredded, dried plant material. It is classified as a designer drug because its chemical structure is similar but not identical to that of cannabis (or marijuana). It is legal in states where cannabis is not because its molecular structure differs from that of illicit drugs. This also means that it has a similar but much stronger effect on the brain than cannabis.
Synthetic cannabinoids, albeit more intensely, act on the very same brain receptors (cannabinoid receptors) as tetrahydrocannabinol (or THC, the active ingredient in marijuana).