What is Paganism?

Can someone please explain what Paganism actually is?
I have minimal knowledge and my girlfriend recommended this site ~ She is a Norseman Pagan.
So please do explain; greatly appreciated

[norseman]

Paganism is a cover-all word for a whole raft of beliefs and life styles. It tends to cover a great deal of ancient belief systems - many Bronze/Iron Age European cultures i.e. Celts, Saxon, Norse, etc.
Best way to think of it is - Catholic, Methodist, Quaker - all different but all Christian.
I follow an ancient path, so that would make me a Primitive Pagan. Others in this forum, follow a modern path such as Wicca and are also pagan.

[Albalida]

Any religion whose believers identify as Pagan is Pagan.

It basically comes from the spread of Christianity going faster in the cities. The people in the rural farmlands, or "paganus" in Latin-speaking cities, didn't convert to Christianity as fast as the city people did. So, "paganus" went from meaning rural villager to meaning not-a-Christian. Basically whenever that happened, and it wasn't only in Italy--you got pagans.

Hellenistic Pagans (from Greece, believers of the Olympians)
Kemetic Pagans (Kemet being the old name for Egypt, so that's Isis and Horus and all that)
Celtic Pagans (I lean more towards Irish with the Tuatha De Danann, but other parts of the British Isles have their own forms of Paganism, and France can count as Celtic too for some reason.)
Norse Pagans (That's Odin, Freya, and the other Aesir and Vanir)
Wicca (who tend to mix-and-match)
Faelatry (that's belief in fairies, which tended to be closer to the earth than the pre-Christian "state" religion of warrior gods in Iceland or Britain)

You'll notice that it's mostly Western-hemisphere. It's a bit of a debate whether Hinduism counts--some Pagans who adopt Eastern religious practices of course make Hinduism Pagan, but many Hindus who have always been straight-up Hindu go like, "Huh? Hindu doesn't need to be Pagan. We're already a religion. We were never uncool." Which is, I guess, a fair point. Many Pagan faiths were "uncool" once--enough that so few practiced it and (except for the stories and myths) the living faith had usually at some point died out or went forgotten. Native American pre-Christian faiths are also somewhere between counting as Pagan and not counting as Pagan.

Pagan is more of an adjective than a religion.

Even in something like Norse Paganism, there's going to be a lot of variety. There's the Asatru, who I gave an example of; the Vanatru, who honor the pre-Aesir pantheon; there's Norse Wiccans who know the mythology but do a bunch of other stuff slightly differently; there's a kind of Norse Paganism that ignores the Aesir and focuses more on ancestral worship and honoring the land wights (like Faelatry with ancestors added)...

[Jenny Crow]

Pagan - - not a follower of the Abrahamic faith group (which includes Christianity).