Technically speaking grass is a herbaceous plant with long thin green leaves which grow from its base. The stuff that makes up your back garden lawn is a type of grass and there are several different types.
If you’re asking about “grass”, then you’re probably referring to a cannabis (otherwise known as marijuana), which is basically the dried flowers, leaves and other bits and pieces from the Cannabis plant. As with all plant materials, it contains lots of molecules, but the “active ingredients” that people take cannabis for are a family of compounds called cannabinoids. These cannabinoids enter the body and bind to specific sites, called receptors, in our brain (and on nerves elsewhere in the body) and have all sorts of effects – the most well known ones are on mood. Why do we have receptors for some plant product? The answer is that there cannabinoid-like molecules that produced inside our own bodies, that also use these receptors to send signals around the body. These “endogenous” (intrinsic, or inside us) molecules are called the endocannabinoids, and if you want to find out more about these then you could google “anandamide”. One of my students (James) is working on how cannabinoids (particularly THC – tetrahydrocannabinoid) affects blood pressure (it tends to reduce it).
How does he get THC? Well, he doesn’t buy it on a street corner – rather, we go to a pharmaceutical company that makes a pure form of the drug, and will sell it to us as long as we keep it under lock and key! We also only use tiny amounts of the drugs in our research, so there are no late-night raves in the lab.
It appears that the other two have pretty much answered your question here Emily, all that I can add really is that ‘grass’ covers a great range of plant life, from grass to wheat to corn to bamboo. What is interesting is that approximately 20% of the earth’s vegetation mass is made up of grass, and growing it is actually a million-pound industry because of the high demand for artificial grass for sports arenas etc. There are almost 10,000 known species of grass!