I think mostly using familiars is idolatry. You are giving recognition, worship, and power to items or animals that don't possess those capabilities and putting them before God in your life. For the most part, familiars would be an item or animal that the "witch" uses to focus power or to cast spells. Some think that they can give their animal a spell and send it to someone and the spell will rub off or some other such nonsense. Witches aren't the only ones who use familiars. Having "good luck" charms that you truly believe have the power to change outcomes would be, in a sense, a familiar item. Having an animal that you give uncharacterstic properties to (oh, my dog tells me when people are sick, my cat can sense when something bad is about to happen, it always rains when my dog turns around three times, etc) would be giving it the properties of a familiar.
The reason witchcraft is wrong is because it puts yourself as the center of your life and you give all credit of power to yourself and not to God who is truly the center of our lives. Having dabbled in it myself, I can tell you, it's a heady thought feeling like you have the power to make whatever you want happen and that you are oh so clever and wonderful because you know words or incantations that defy reality. Even though I wasn't casting evil spells, or harming anyone physically, I still was deluded that somehow the elements (which God created) would do whatever I told them to do. And yes, things worked...there is always the probability that something will happen after you've cast a spell that you can construe as your spell working. MOst of the time it's just coincedence, but there were also so very scary times where I'm sure Satan himself was present. Sometimes it would get so scary, I finally decided to stop (example, I was mediatating, trying to almost astral project, and something hovered over me and talked to me, and I could feel that it was evil. It scared me so much, I can't even meditate like for yoga anymore)
Wizards I think might be a horrible translation simply because our modern language does associate them with a fantasy being. But the wizards spoken of are probably people trying to use spirits or their own personal "powers" to shape reality and affect the world. It could even mean those who seek to harm people through the use of nature (herbal potions, poisions, etc).
Anyway, those are my thoughts on the three subjects!