Pagan: a very loose and rather broad definition would be "someone who walks in harmony with the Earth and all Her creatures." Paganism is a very old religion/spirituality (to a point, it was humanity's first) and rather widespread. Anyone who respects the Earth could be considered Pagan. No, it is not about the Devil and Hellfire and all that stuff that the fundies will tell you. The word "Pagan" comes from the Latin paganus, meaning country-dweller, just as a heathen is someone who resides in the heaths. Funny how words change over time, isn't? At any rate, if you meet someone who is a self-identified Pagan, that does not mean that s/he is a godless communist who wants the soul of your firstborn child. It simply means that s/he walks with the Earth, and doesn't tramp all over it, "terrorizing the other occupants and kicking over furniture" (to borrow a line from Terry Pratchett).
Paganism: this is pretty much explained above. It is the spirituality that a Pagan practices or is involved in. Some may argue that it isn't so much a religion but a way of life--then again, and to use a sweeping generalization, all religions are (or are supposed to be) ways of life. It includes (not limited to) animism, deism, shamanistic walks of life, shape-shifting, Wicca, and Witchcraft.
Witchcraft: Don't run. Stay calm. I won't curse you if you do run, but it would over all be better for your mental health if you read the following: Witchcraft is not evil. I repeat: Witchcraft is not evil. It is a branch of paganism, younger than the more broad one. Witchcraft is specific. It is more practice than religion--it is a set of techniques to direct and use will to manifest desire. Or, in layman's terms, a set of techniques to make magick. Not all who practice Witchcraft are Wiccan, and not all who are Wiccan practice Witchcraft. Some Wiccans call themselves Witches, and some Witches call themselves Wiccans. I, personally, call myself a Witch. I like the sound and power of the word. Witchcraft includes sets of skills and techniques you can learn, and doesn't necessarily involve faith in a particular set of beliefs.
Wicca: A fairly new religion that falls under the heading Paganism and is closely related to Witchcraft. It has a set of beliefs and is more religion than practice. Wicca is a fast-growing religion in the US and the world, and most likely at least one person you know is a Wiccan. Unless you don't know anyone, and you know you are not a Wiccan. In that case, I throw up my hands in defeat.
Solitary Witch: A Witch/Wiccan who practices magick on his or her own, without the help of other Wiccan/Witch friends (see next).
Coven: A group of Witches/Wiccans who do magick and ritual together to accomplish a common goal.
Magick: This is a tricky one. There are many different definitions. The closest I can come is "focused will to manifest a desire." Basically, concentration=acquiring your desire. There's more to it than that, of course--there's casting a circle to keep the evil spirits out (it doesn't hurt to take precautions), there's raising energy, there's different types of practicing magick, etcetera. It's too much for me at the moment, but I promise I will later put much more up on the subject. You may notice that the magick I speak of is spelled with a "k" on the end. This is intentional. "Magic" refers to magic practiced on stage--illusionary type stuff that you see on TV with the "magician" and his scantily-clad "assistants." "Magick," or "majik" refers to the kind practiced by Witches/Wiccans/Ceremonial Magicians. It actually does create change in the world--and even the simplest spell can be quite powerful.
The Pentacle (pentagram): There is some argument as to the 'correct' name of this five-pointed star. Pagans can't agree on anything, I find. I prefer pentacle, and so will use that term. The pentacle is a five-pointed star surrounded by a circle. For years the general public has regarded this as a Satanic sign thanks to some rather unfavorable propaganda regarding Witches. People seem to think that Witches worship Satan--well, a 'Satanic Witch' is an oxymoron, because the Satanist's code of ethics is completely opposite to ours. The pentacle inverted, with the two points at the top, can and usually is taken for the sign of Satan. It can also denote a different level of initiate in a coven--I forget which level, because I frankly just don't care that much. Just remember that the pentacle right side up, with one point on top, is the symbol of the five elements surrounded by the love of Spirit. Some people prefer the inverted pentacle because to them it symbolizes the feminine spirit. I personally avoid using it, as it is usually associated with Satanism and I feel no particular connection to it. To each his own.