There is no difference between "null" and "void" and to understand why it helps to know a little about the history of the common law of England.
After the Norman Conquest, the law was written for the most part in French and Latin. By the 1500's the law was written in French and English. There was a fear that by using the French word only meaning which lay in the English word would be lost, or used as a loophole, thus both words were used. Null and void, to have and to hold, to cease and desist are all examples of this tendency towards parallel construction in legal writing. The words "valid" and "invalid" refer to a wholly different matter. You might have an invalid provision in an otherwise valid contract, such as a provision that the parties agree that mandatory overtime rules will not apply. Keep in mind also that there are voidable contracts, such as a contract entered into by a minor. Such a contract has valid clauses but can be voided by the minor at any time (this is somewhat of a generalization).
I don't know what @Yosef Baskin meant by, "for a valid contract, a judge can later deem it void due to actions of either party." A party's later actions may constitute a breach of contract, but a breached contract is not a void contract. A contract whose object is illegal, such as murder, is void, or null and void. An otherwise valid contract is not made void by the subsequent performance or non-performance of the parties. A change in the law may void a contract in whole or in part, such as a law prohibiting the sale of a particular commodity.
