Some of the "legal herb" is just other plants that look and smell a little like MJ, but have no pharmacologic effect whatsoever.
The K2 "legal" weed and similar products, consist of a borderline-legal synthetic cannabinoid that is sprayed onto a legal herb for smoking in order to mimic marijuana.
Never tried it, and have no interest in doing so, but I've heard that the effects are somewhat similar to the real McCoy, only inferior in quality and duration.
Unlike the real thing, which has been in use for at least 4 or 5000 years of recorded human history, these legal analogues have only been around a relatively short period of time, so track record on safety is lacking.