I'm afraid I know little about Asatru. I think its adherents worship the Norse gods. I only know of one person (in the real world and the Internet) who practices it, so I think it's pretty obscure, even among the other Pagan religions. Paganism and Neo-Paganism are pretty broad categories that cover a number of Pagan religions like Asatru and Wicca. I'm afraid I don't know anything about Aryanism or the Aztec religion (I'm not aware of anyone that still follows it), but I can tell you a little about Wicca though, having been somewhat involved in it and similar spiritualities and religions when I was younger. My knowledge is getting pretty rusty though. After I was saved I dropped it all and threw out my books.
If I were to summarise Wicca, it would be like so:
Wicca is a modern religion invented in the 20th Century by Gerald Gardner, with inspiration from the old Pagan religions. Today though, it's little more than a mish-mash of stuff from all kinds of religions, Pagan ones in particular. There is no real "structure" to the religion, and no authority, the only exceptions perhaps being the "Wiccan Rede" and "Three-fold Law". The most often quoted part of the Rede is "An it harm none, do what ye will". The Three-fold Law essentially says that whatever you do in Wicca will come back to you magnified by three - if you do "white magic" for example, you will receive the benefits back. If you do harm somebody (ie black magic), you will receive three times whatever you do to somebody else. Thus harming people magically is supposed to have dire consequences. Wiccans are essentially free to pick and choose their beliefs as much as they want. As you might imagine, it is very relativistic - there's a real "true for you but not for me" mentality. That's how you end up with "Christian Wiccans" and such. Wicca is closely associated with other Occult practices like witchcraft, Kabbalah, divination, etc.
The usual Wiccan gods are generally referred to as the Lord and the Lady, though I know little about them. Some Wiccans don't even worship these gods, and instead steal gods from other religions. Some Wiccans for example worship the Greek pantheon of gods. In addition to these, Wiccans often also worship nature to some extent. Wiccan practices typically involve various rituals and magic. If I remember correctly, magic in general isn't particularly different from the Harry Potter stuff, and the rituals are more like a practical combination of prayer and magic - ritual items are used and incantations are made.
In my experience, Wiccans are very defensive towards Christianity, and even bitter towards it. Many see the Salem Witch Trials as persecution against either their faith (which, as above, didn't exist until the mid-20th Century) or the faith of their ancestors. They're also bitter that Christians portray them as Satan-worshippers, which they vehemently deny. And to be fair, they don't worship Satan any more than other false religions do. There also seems to be a mentality that their religion is superior to the Judeo-Christian faiths because they may claim that Wicca is derived from ancient religions and practices pre-dating Judaism. I am very sceptical of this argument though because even if it were true that its practices are derived from these ancient religions, none of them are even remotely similar to Wicca or even witchcraft in general as they exist today. If you will be evangelising to these Wiccans (and the other Pagans too), these might be obstacles you'll need to overcome. Refuting the idea of relative truth and/or relative morality might also come in handy.
That's about all that comes to mind for now. I can answer more specific questions about Wicca and Paganism in general if you have any. If you wanted to learn more about these religions, Wikipedia and reliable Christian sources (CARM has info on Wicca if I remember correctly) are probably the best way to go, though sites that portray Wiccans as actively worshipping Satan probably aren't particularly reliable.
