it's a belief, if you don't believe a spell will work, then it won't. some witches [like Brysing] don't believe in magick i.e. waving a wand over a candle. magick can be seen as a number of things, ancient times the ability to cure people with herbs was considered a form of magick. science could be argued as magick, but i tend to see magick as a parallel to science, something that has yet to be defined. funny enough, i have read in many different books and sites, that many scientists and doctors follow a pagan path. hopefully this is true, i like this 'fact'.
magick in the religious sense is a natural energy in everyone and everything. to cast a spell, we tap into this natural energy, charge it and send it out to help create desired change. since it's a part of nature, however, it cannot do things that go against nature [like invisibility or teleprotation] it also works with energy and cannot conjure instant physical change. [transformation]
energy is all around us, it is inside us, and this energy can influence us in ways we might not realize. the energy around us is neutral, once it comes in contact with us it is given a positive or negative charge. too much positive or negative energy inside our bodies can be a bad thing, a build up can make us sick, we could also unintentionally curse ourselves or someone else. study about magick to decide for yourself what you believe, as well as how and why it works.