The question "What is Paganism?" is almost too broad to answer.
Simplistically for someone at the beginning of their understanding it may be taken as an umbrella term religions and/or belief systems that venerate nature and/or pre-Christian Gods and Goddesses, such as those of ancient mythology.
The nature of what it is to be Pagan is such that true insight as to what it means to the individual can only be gleaned from treading the path oneself. Paganism allows for a direct link between the individual and the Divine, without intervention. In essence each Pagan is capable of being their own priest or priestess even if they choose to delgate this capability to another through membership of a coven or other such working group.
Paganism covers a variety of paths including Wicca, Druidry, Heathenry, Shamanism, Traditional Witchcraft and many others. Indeed many solitary practioners choose to borrow, hopefully sympathetically, from more than one tradition taking elements which suit their personal beliefs and moulding it into their own eclectic system.
Hopefully this will give some idea of how difficult it is to speak for more than oneself on the topic, and in turn why this initial description of such a broad topic is so brief!!