Well... I'll try and simplify it. Basically, these are referring to (if I understand correctly) when different "eras" of comics began. Different styles in storytelling, entire rewrites of characters (DC), and different art styles. The way I see it, the bridge between the golden and silver ages is the easiest: by the end of the Golden Age, thanks to many critics ("the seduction of the innocent"), comics were toned down and frowned upon. Something caused a great revival, thus the Silver Age. As for the modern age, the DC Crisis began that - it rewrote their entire universe.

I hope I've helped, and am not full of misinformation.