There was a question the other day regarding how a certain new spell that I posted operates. To answer that question more generally, I point you to this excellent article on the Laws of Magic. While you may enjoy reading that article, here is a brief summary. The following notes should assist in the creation, and prediction of the effects and limitations, of spells employed in Dungeons and Dragons (understanding, of course, that those spells are simply game mechanics, and are not real...).
Law of Mind Over Matter: This Law holds that the mind controls the body and the physical environment. Put another way, every physical creation is preceded by a mental creation.
Law of Belief: this law states that in order to command or perform magic, you must believe in it, and your power to control it.
Law of Knowledge: knowledge is power. The more you know about a person or phenomenon, the more powerful you magic can be in relation to them or it. Spell and Character "levels" in D&D try to account for this law: for example, higher level characters can use higher level spells of the same type that are more powerful (think of the power word spells, or the various fire-control spells).
Law of Self-Knowledge: Nosce te ipsum -- know thyself. Those who are self-aware and know their abilities are better able to use them.
Law of Names: knowing the name of something gives you power over it. The more specific the knowledge, the more power you have. For example, Maple heartwood is more specific that wood; Thomas, son of Donald, is more specific than Thomas, is more specific than Man.
Law of Association: this law states that things that are associated with each other can effect each other. An obvious example of this law would be a voodoo doll: a pin poked into a voodoo doll will injure the person the doll is associated with.
Law Of Similarity: this is a sub-law of the Law Of Association, which posits that effects resemble causes. For example, pouring water on the ground will make it rain, or striking a match will create a fireball.
Law of Contact or Contagion: this law states that things that were once in contact will continue to act on each other once the physical contact is severed. A good example of that is the "Leap Of Logic" spell that I posted the other day: the stone that the player throws, continues to be "connected" to the player after it is thrown, allowing the player to leap to the same location as the stone. The author of the above article uses another example, of a warrior eating the liver of a lion, in order to gain the strength of the lion.
Law of Words of Power: Words of Power are such things as "abracadabra", hocus-pocus, Jehovah, Pope, Vizier, or the proper name of an infernal Lord. The power of the Word is tightly bound to the weight to which others ascribe to the Word, or the person using it. This is why it is important that magic-users garb themselves in sufficiently arcane and fearsome attire, that way, their use of the "power word" spells will have greater effect.
Law of Identification or Imitation: this allows someone to assume the characteristics of another, for example, shape-changing, contact other plane, or divination spells.
Law of Synthesis or Opposites: this law states that two opposing viewpoints can be synthesised into a new viewpoint that is not simply a compromise between the two.
Law of Polarity: this law states that everything can be separated into two polar opposites.
Law of Balance: this is an exhortation to be even-tempered, flexible and open to alternatives
Law of Infinite Data: this law says that there is always more information out there.
Law of Finite Senses: while data is infinite, our capacity to receive and process all that data is limited by our senses.
Law of Infinite Universes, Law of Pragmatism and Law Of True Falsehoods: these three laws are inter-related, suggesting that, since everyone sees things differently, one should be flexible and accepting when your views conflict with the views of others.