Same thing. Cannabinoids are the class of chemicals to which thc (tetrahydrocannibinol) belongs to, thus thc is an example of a cannabinoid.
Messengers. Thc is a cannabinoid. Cannabinoids are a class of chemicals that often act as messengers within the cells of animals and plants. Thc is a plant based cannabinoid but there are many others, including some that occur naturally in humans. They all serve to deliver chemical messages from cell to cell. Some are even used as medicines.