A gleba is a solid mass of spores, generated within an enclosed area within the sporocarp. As the spores mature, the sporogenous cells often liquify and/or disintegrate, leaving just the spores behind as a powdery mass that can easily blow away (as in the picture). In other cases, the gleba may be sticky, as in Sphaerobolus stellatus; or it may be enclosed in a case (called a peridiole), as in the Nidulariaceae.
A mazedium is a special kind of gleba found only in certain ascomycetes like Onygena equina. I'm not sure why it is so distinctive that it can't just be called a gleba; maybe the Basidiomycologists and the Ascomycologists weren't communicating very well when the term was coined.