As you might have suspected, I'm not a real doctor.

I'm new to 3D coding and have just begun a search for the best 3D API available. Now that I have found OpenGL, I find myself with another problem. Cosinus and Sinus are two mathematical functions wich are used in 3D Graphics extensively, yet I don't understand what exactly they do. I feel that once I master this, I will know how to draw spherical and circular objects in OpenGL, and be able to rotate objects in 3D on the screen. I never whent to school, so it's not realy something I could have done something about. Can someone, anyone, give me a brief explanation of what exactly the use is of these two functions, and why they keep popping up in 3D code?

Thank you.