They both are spiritual paths that follow a nature tradition, and apart from labels, there is not a great difference between them these days. One might argue that Wicca contains more magical rituals and has a tighter struture, whereas paganism is more electic and 'lighter' in comtent.
"Pagan" Is a an umbrella term meaning any religion that is not Jewish, Muslim, or Christian. Wicca is Pagan, there is no such thing as "traditional Paganism" etc. like my claim there is, Pagan is an umbrella term, always has, always will be.
This is a complicated question, with an equally complicated answer. We have to first understand the evolution of the word "pagan". It initially came from a Latin word meaning "country dweller", and was used to describe the people in the countryside who kept to their old ways instead of converting to the new religion (Christianity), like the people in the cities. The word was used in a derogatory sense and is akin to the modern-day word, "redneck".
Over time, the term came to refer to anyone who was not of an Abrahamic religion (namely, Christianity, Judaism, and Islam). However, it's *more* specifically referring to the indigenous spiritualities of various lands (think: Native American spirituality; Shintoism; the Druids; Norse/Greek/Egyptian/Celtic/etc mythology; et cetera). While yes, many religions may "technically" fit this definition, most probably wouldn't call themselves "Pagan", as the term "Pagan" is rather Euro-centric.
Modern, Neopaganism is, in general, the attempt to bring back those ancient indigenous spiritualities, and adapt them to the modern world. Some try to reconstruct the ancient religion exactly (to the best of their knowledge on recorded history), some bring back many of the basic beliefs of those ancient religions and mold them to fit into the modern world (including adding beliefs), and some take elements of many different indigenous spiritualities, along with modernism, and create an entirely new religion.
Common themes among Neopagan religions tend to be:
1. Polytheism, Pantheism, animism, and similar beliefs about universal energies and spirit beings.
2. Some sort of belief in a form of "magic", divination, spiritism, shamanism, witchcraft, etc.
3. Reverence for nature, the natural world, nature-oriented deities, and holidays or sabbats that are based on the cycle of the seasons. This is not necessarily "nature worship", but rather a respect and connection to nature.
So, while many religions may fit the definition of "non-Abrahamic", generally, one who uses the term "Pagan" or "Neopagan" is one who follows a belief system that probably has ancient (pre-Christian) religious connections, and tends to fit into the above bullets.
There are many Pagan/Neopagan religions and spiritualities. Wicca is just one of many.
