Perspective Projection with OpenGL

I am confused about perspective projection.

Here is the scenario that is confusing me. My frustrum's front plane is basically positioned at at positive z-axis and the back plane at a negative axis and rotated about some deg along the posive z-axis.

Now, whenever I go through examples I see that the near and the far plane are all present in the negative z axis.

My approach in this case was basically something like:

glMatrixMode(GL_PROJECTION);    
glLoadIdentity();
glFrustrum(-xval,+xval,-yval,+yval,-10,+10);
gluLookAt(); // some point lying on this axis of symmetry about the point
glMatrixMode(GL_MODELVIEW);

So, in the above case what is the behavior that I should expect for the glFrustum with a negative as well as positive value for z. i.e. in this case -10 to +10.