The very fact that that this question needs to be asked is proof of the immense strength and power of prejudice and misinformation. Asking the difference between witches and satanists is like asking the difference between yellow and purple, or the difference between up and down.
While some satanists may call themselves witches and some witches may call themselves satanic witches, both are practicing their own combination of the two faiths. But it's no wonder that this misinformation is so embedded in the subconscious mind. Witches were only associated with satanism starting around the time of the Inquisition. The Christians were working very hard to convince everyone in all of Europe to covert from their Pagan beliefs. The best way to do this was to spread lies and misinformation about the beliefs associated with witchcraft. Suddenly the devil shared striking similarities to pagan gods, and accusations of having midnight meetings with Satan abounded. This is what sparked the witch trials and was the real purpose of the Inquisition, to quash anyone practicing witchcraft from the face of the earth. Unfortunately, even after several centuries, witchcraft has been unable to shake its incorrect association with the satanic. Here are just a few of the ways that the two are completely different and ultimately like comparing oil to water:
1. Satanists fall into one of two crowds. Either they do believe in the Christian devil and by inverting all Christian symbols and rituals pay allegiance to this entity, or there are the class of Satanists that don't believe the devil exists as an actual being, but that the devil is really symbolic of our own inner ego and desires. Practitioners of witchcraft don't believe either of those things. They believe that the devil is a creation of Christianity to scare people into behaving by threatening them with this menacing character. While you may hear Christian fundamentalists tell you that witches worship a devil-like character, this is untrue. It is true that many witches honor a Greek God by the name of Pan. Yes, he does have horns and is generally portrayed as being half man and half animal. Sound familiar? Pan is a God of the fields. He presides over shepherds and their flocks. He is who a witch may pray to in order to protect their flocks. He was the most popular God around the time of the Inquisition, as most of the peasants were farmers, so it's little surprise that Christianity chose him to base their devil off of.
2. Satanists believe that they are their own highest authority, that they should bow down to no one, neither man nor God. That and weakness within themselves is considered the ultimate sin. Witches definitely believe in a higher authority and that God and Goddess are there to guide us in the way we should go. But they stop short of actually telling people what to do and what is right.
3. Satanists admire that all human beings are capable of and have the capacity for desires like spite, lust, envy, and greed. They believe these desires should be indulged and not curbed, and that man has the authority to do whatever he pleases, but not to sow anything that interferes with another's free will. So a few acts like rape or murder are disallowed. Revenge and materialism are celebrated and encouraged. Witches follow one very simple, yet very powerful rule: If it harm none, do what ye will. Seems simple, but it's very all-inclusive. You may do whatever you like as long as you're not harming another person either physically, mentally, emotionally, or in any other capacity. This even includes unintentional harm. Witches generally believe that a simpler life is a life closer to God and Goddess and that materialism and greed are something not to be fostered.
These are probably the biggest differences between these two groups. There are dozens more that could be included, but these are the basic tenets of belief and therefore the most important to consider.