Paganism is any non-Abrahamic folk/ethnic religion. Its just whatever Christians want to call people not like them so they can feel better about their crusade of bloody mass murder. At least that is how it used to be used. Its a less murderous crusade now but I didn't forget.
people seem to forget that Paganism ( in all of it's forms) is WAY older than Christianity, and Christianity actually absorbed some pagan beliefs-for instance wedding rings,wedding ceremonies, funerals (putting flowers on graves, and embalming), pagans also created statues, US money has pagan designs in it ( the eye over the pyramid), the different days of the week for different Gods. So really it's paganism in all of it's forms that inspired christianity . you can read more here http://articles.exchristian.net/2003/04/is-christianity-based-on-pagan-roots.php
Paganism is an umbrella term that encompasses all "non-Abrahamic" religions (in other words not Christian, Muslim or Jewish). So in and of itself there is no unified "Pagan" religion, there are just a multitude of religions that fall under that umbrella - Wicca, Asatru, Hellenismos, Kemeticism, Eclectic NeoPaganism, Celtic Recon religions, etc... I would suggest reading the following for a detailed overview - http://theinformedpagan.org/what-is-paganism/