Okay. The first compound, the Delta-9, is the reason you smoke marijuana. That produces the desired affects, the euphoria.
When it is in your body your body looks at it as a foreign substance. So it starts to metabolize or alter it chemically. One of the things it forms is the 11-Hydroxy-THC.
This is also a psychoactive compound, but rarely do we find it because it is so short lived in the human body. And then it goes on further to be metabolized into the carboxy. We call it carboxy because the full name for it is 11-NOR-Delta-9-Tetrahydro-Cannabinoid-Carboxylic acid.