Can someone explain the difference? I searched around the web, but didn't find too much about the difference of these two beings.

Also, what does this line from the UESP mean: The Magna Ge are now synonymous with the stars, which they created.^{http://www.uesp.net/wiki/Lore:Magna_Ge#cite_note-WoT-1} http://www.uesp.net/wiki/Lore:Mankar_Camoran credits them with creating http://www.uesp.net/wiki/Lore:Mehrunes_Dagon in "the bowels of Lyg"

I can't really answer your initial question, or your second. The quote confuses me. Wasn't Lyg the previous Kalpa's Dreugh-world-thing? I've read some discussions about Mehrunes Dagon being Magnus. Maybe it's saying the current Mehrunes Dagon is the past Kalpa's Magnus? Would make the Magne-Ge's involvement somewhat less strange. But... They're supposed to be his children, not the other way around.

Off the bat, I wouldn't give anything Mankar Camoran says too much thought. He is a raving madman afterall. Some of his ravings are certainly interesting and almost plausible, but they're still just ravings.

Concerning the difference between Magna-Ge and Celestials (I assume you're referring to the ones encountered in Craglorn), I tend to see the Magna-Ge as very deistic divines. Gods who have no interest in the affairs of mortals. Maybe they did at one time have some interest in the grand plan to forge the mortal plane, but once they realised the great sacrifice required to enable its birth, they got the heck out of cosmological Dodge and haven't looked back since. The Celestials are obviously more willing to get down and dirty on Nirn than the Magna-Ge ever were.

Perhaps the Celestials are divines similar to (or even former members of) the Magna-Ge who decided, for whatever reason, to abandon the mainstream practice of non-participation in the Mundus and moved to get more involved in the world of mortals. (This is what I imagine being the reason for Meridia's alleged expulsion from the Magna-Ge pantheon. She wanted to get hands on helping mortals and as such was cast out of Magnus' divine 'Nope.avi' club.)

Or perhaps the Celestials are the incarnation of the Magna-Ge in a non-Aldmeri-Cyro faith. One of the Celestials commands an army of ancient Yokudans, so maybe the Celestials are to the old Yokudan faith as Tsun is to Zenithar. Dovahkiin who enter Sovngarde see Tsun in his Nordic visage regardless of their race, so maybe we all see Celestials in that form because they were compelled to reveal themselves because of non-Aldmer beliefs?

So if the Celestials are Gods and if they didn't help with the creation of Nirn (I assume they didn't), wouldn't that make them equal in power to Daedric Princes?

Errrr...Mnemoli, the Blue Star?

I don't think she's your average Magna-Ge. The Ayleids believed she, alongside Meridia and http://www.reddit.com/r/teslore/comments/27ak1h/fragment_a_treatise_on_xerolyg/, was one of the "http://www.imperial-library.info/content/exegesis-merid-nunda."

"http://www.imperial-library.info/content/forum-archives-michael-kirkbride"

"http://www.imperial-library.info/content/vehks-teaching"