Magic can generally be categorized as sympathetic or direct. Sympathetic magic acts upon symbolic representations of the target of the spell. Direct magic acts directly upon the target. The most frequently used form of magic is sympathetic. J. G. Frazer delineated in his magnum opus The Golden Bough two principle methods by which sympathetic magic can be described as working through (1).
The first he called the Law of Similarity. This law argues that an effect resembles its cause and that like will attract like. This is often summarized in the Hermetic axiom 'As Above, So Below.' The second principle method he called the Law of Contagion. This law argues that what is done to a material object will influence the target of the spell once they come into contact with said object, and that effect will continue after initial contact is broken (2). Most sympathetic magic is a combination of the two laws being put into effect.
Direct magic is the application of one's efforts to the target in question. Herbalism and most 'potions' are direct magic. The practice of laying on of hands and direct manipulation of one's bio-energy field can be considered direct magic as well. One could argue that direct magic is a function of the Law of Contagion. I would contend that the Law of Contagion can only be applied to items that are enspelled with the intention of having an action upon some one when handled or otherwise coming into physical contact with said person (3).
1. Full discussion of this can be found in chapter three of the condensed version of the Golden Bough. Specifically, reference section one, paragraph one.
2. One could argue that the Law of Contagion was something of a precursor of the theory of quantum entanglement as described in the EPR paradox. It is important to keep in mind that the development of modern physics was not established until after J. G. Frazer's work was published. One must also consider the author's bias against magical thought.
3. People are the most frequent targets of spell craft, thus my examples revolve primarily about them.