A patent claim is a sentence at the end of a patent application that describes what the applicant asserts to be the invention. There can be a number of different patent claims, but each is only one sentence long. As you can imagine, a patent claim describing a new type of ultrasound machine, computer network, or process of doing surgery is going to have more than a dozen distinct nouns and more than a dozen distinct verbs. So this “sentence” with a single period may have a number of paragraphs within the sentence as patent attorneys will separate parts of the claim by semicolons and white space.
The patent claim is the payload of the patent application process. In order to hold someone liable for infringement, one needs to have a situation where the accused infringer has a good or service with every noun in the patent claim and every verb in the patent claim.
It can be useful to think of each noun and verb or other requirement in a patent claim as links in a chain. An accused infringer needs only prove the lack of one link in the chain (lacking one noun, verb, or other requirement). Thus, part of the work of drafting patent claims is trying to find the least number of required elements that can make it through the patent process so that it is difficult for an accused infringer to escape the scope of the claims.
Some claims build upon other claims. Thus claim 3 may say the process of claim 1 further comprising step X and step Y. Claim 1 would be an independent claim as it does not depend on the content of other claims. Claim 3 would be a dependent claim as it adds onto another claim. Claim 4 could depend from dependent claim 3 to add additional requirements and thus further narrow the scope of the claim. While independent claims are the most important claims, as one cannot infringe a dependent claim without infringing the independent claim, dependent claims can help define the scope of the invention and can survive arguments that the broader independent claim is too broad.
Aside from the distinction between dependent and independent claims, perhaps the most basic distinction in types of claims are claims that describe a thing and claims that describe a sequence of steps. The claims that describe a sequence of steps are sometimes called method claims, process claims, or sometimes get written to describe a sequence of steps performed by a computer and are called software claims. The names really do not matter as the concept is that you are claiming a sequence of steps.
Other claims describe a thing. The thing might be a novel material, chemical, or drug. Frequently, the thing may be a manufactured item. Sometimes the claim may focus on a combination of things put together into an assembly. Sometimes the thing may be a set of things that are sold together in a kit.
In order to make it difficult for a competitor to try to escape the scope of patent claims, a patent attorney may seek a combination of claims including some on the sequence of steps and some on the item including the material the item is made of, the item by itself, an assembly that involves the item, a kit that includes the item, or even a kit of components that can be used to make the item.
The preferred format for claims in other systems differs from the preferred formats here in the United States so there can be work to revise the United States claim sets into formats that work in other patent systems.