In most countries in order for a patent to be valid the invention defined by the monopoly claim must be novel, include an inventive step and be capable of industrial application. Also, some subject matter is pre-determined as not being patentable - for example methods of doing business and computer programs. However, the nature of these exclusions is complex and may not prevent protection if the patent application is properly presented.
Novelty means that exactly the same thing (more or less) must not have been "made available to the public" anywhere in the world at any time before the patent application was filed".
An inventive step is said to be present if the difference over what was known before is more than a mere workshop development. It has to be something which would not have been obvious to a person of ordinary skill in the technical field concerned.
Industrial application simply means that the invention must be capable of being made or used in any kind of industry.
The validity of a patent can normally be challenged in the Patent Office or in the Courts but detailed rules apply.