Merry Meet savannahlove
A witch is anyone who casts spells. Even Satanists and Christians could be considered witches if they cast spells and do magic, but they aren't Wiccan because they follow a different religion than Wiccans. A Wiccan is someone who follows the religion of Wicca, which is an officially recognized religion just like Christianity, Judaism, Islam, and Buddhism. If you get nothing else from this, remember that "Wicca" is the name of the religion and "Wiccans" are the followers of Wicca. So you wouldn't say, "I have a friend who is Wicca." Instead, you would say that your friend is Wiccan.
According to the Wiccan Rede, which are the rules that Wiccans abide by, Wiccans are not supposed cast harmful spells. The Wiccan Rede says, "And ye harm none, do what ye will."
Someone who claims to be a "witch" but who is not "Wiccan" is not following the religion of Wicca. Therefore, they do not have to follow the rede. So they can throw curses at you from across the room.
If you think you want to be Wiccan, or you want to know the difference in Wicca and witchcraft, here are some useful pages to read that explain the difference: