NeHe tutorials

I''ve started on the NeHe OpenGL tutorials and i''m worried that all i''m doing is copying code without really understanding why i''m doing so, so i went to the msdn site and read everything on openGL for some background information. However even then i understood the basics but was still confused by some areas due to a lack of understanding in 3D maths and terms. So then I tried looking for some tutorials that would explain the basis of 3D graphics like what clipping rasterizing etc.. involved why i had to clip a scene what z buffering did what b-splines are etc... But I can''t find any so could someone post a link to some sites with good background information on 3D graphics (not programming just general 3D information)