The answer to that question is both simple and complex. The simple answer is that Paganism is an umbrella term under which you'll find religions based upon pre-christian beliefs, with a focus on Earth based spiritually & and the practice of magic as a way to tune into the natural rhythms of the Earth.
These faiths include but are not limited to Wicca, Witchcraft (with or without the spiritual component), Druidry, Shamanism, Goddess Worship and Feminist Spirituality, Heathenism, Pagan Reconstructionists, Polytheists, Panentheists, Animists, Nature Mystics and even Ceremonial Magicians (though not all of the individuals who practice these call themselves "Pagan").
Although indigenous European religion has dominated the modern Pagan movement, many have expanded the umbrella to include indigenous religious practice from the America's, Africa and Asia, so now it is not uncommon to find Pagans who worship the Gods of Hinduism, Buddhism, Voodoo, Yoruba, Santeria and the plethora of Native tribal faiths. In a multi-cultural world, Paganism honors diversity in all its forms.
The complex answer (imbedded in the simple answer) is that Paganism is a wide spectrum of beliefs rooted in the personal and experiential; therefore highly individualized.
Paganism has no central hierarchy or book of law, but is an ever evolving conglomerate of ecstatic rites intended to bring the practitioner into a union with the sacred, the energy and cycles of the Earth and the Gods or Goddesses that the individual finds holy.
Most Pagans believe in the sacredness of the Earth herself, and that all life on Earth is interconnected and divine; and so it is common to see festivals and feast days centered around the attunement with the seasonal tides of the planet.
For many the Gods and Goddesses of the pre-christian era are immanent in the world, others find them to be symbolic archetypes for understanding humanity.
Unlike other religions, Paganism at its heart is about joyful co-existance on Earth and with Earth; it is about the fertility of the land as it is linked with our own lives (whether we are procreating or not). Our rituals are not only spiritual and profound, they are celebratory unions with the sacred and for those who honor these sacred unions, deep connection, transcendance and tranformation is the reward.