In the show Supernatural, the Winchester brothers frequently cast various spells. Summoning angels or demons, warding angels, binding demons, etc. are all spells that they often use in their work. However, they also hunt and kill witches, and witches seem to only be differentiated from regular people by their use of spells.
So, what is it that makes witches witches, and the Winchesters not witches?
The primary difference between the Winchesters and Witches is about their choices made when using magic and against whom the magic is targeted. In the strictest sense, the magic the Winchesters use is a form of ritualistic magic, able to be done by anyone with the correct incantations, material elements and making the correct sacrifices.
What separates them is their choices of targets, how they amplify their power and the means by which they operate when using magic. Black magic is designed to harm, curse, hex, torture or under the right circumstances kill the target. The Winchesters almost never use magic in that fashion. They prefer most often to use magic for circles of protection, obfuscation, summoning and occasionally divination.
Being a Witch has more to do with your:
Choice of magic: Witches have been known to have sold their souls for the power to work greater magic against other humans. This magic is called Black Magic and is usually more potent than anything a non-aligned practitioner such as the Winchesters can use.
Source of magic: Human torture or human sacrifice are common means of enhancing a witch's power and several have been seen to do so during the series. Don Harding and Tracy Davis - begin sacrificing people in order to raise the demon Samhain from Hell. (S4E7 - It's the Great Pumpkin, Sam Winchester). Witches have also been seen to use a magical tome known as a Book of Shadows or other Grimoire.
Target and intent of the magic: In the simplest sense, ritualistic magic, the sort of magic used most often by the Winchesters can be done by almost anyone, with the right incantation, right elements and sacrifices.
The Winchesters might be considered "good witches" since they do not use their magic against other humans only creatures allied with darkness. Proof of at least their ability with ritualistic magic exists when they create "hex bags" to prevent Lilith from finding them.
During the show it has been made apparent that anyone can practice magic1. The show also goes to great length to explain that witches have greater knowledge and power over harnessing mystical forces. Since Supernatural established that fact that's all that really separates the Winchesters - or more generally speaking, any normal human - from witches.
This issue also serves as a prime example of specialization versus generalization. Almost anyone is capable of practicing magic but witches specialize in it and are, therefore, more capable. Even though there are powerful witches like James Frampton2 who only recently became a witch, it's an established fact that usually the most powerful witches are the oldest ones3.
1 According to the show, witches have existed for many centuries. It's also been stated that the source of their power is mostly demons but it is possible to learn witchcraft without a demon's help. That's not even taking white witches into consideration who surely don't get to practice powerful magic by selling their souls.
2 8x15 Man's Best Friend With Benefits
3 Patrick (5x07 The Curious Case of Dean Winchester) or Don Stark (7x05 Shut Up, Dr. Phil), for example.
I suppose the difference would be that witches use magic regularly, and usually to hurt people; the Winchester brothers use it to save people.
