Cannabis contains over 50 different cannabinoids. The function of most of them is unknown, but a number of them are known to interact with THC (the main active substance in cannabis) to produce different effects. Different strains can have remarkably different ratios of cannabinoids, meaning the highs you would get from smoking them can potentially be very different. Some sativas are known to have stimulating, energetic highs. Depending on the strain, it can increase creativity, amplify sensory input, produce euphoria or paranoia. Jack the Ripper, for example is known for having a very paranoia-inducing high. From what I read, Mekong High gives a clear, euphoric high.