Some of my new inventors have trouble grokking how patent claims work. This is an analogy that I’ve found to be useful.
The claims are the critical part of a patent. You have to get them right, and getting them right is the main reason for hiring a professional patent attorney.
Imagine that your patent claim is a shopping list. Let’s say your invention is a delicious sandwich, and you have “bacon,” “lettuce,” “tomato,” “white bread” and “mayonnaise” on the list. (Maybe you also have a method claim that involves a toaster!)
During the first, patent-getting part of the process, an examiner takes your list and goes “shopping” at the Prior Art store. That is, he looks in the prior art (mostly old patents, although textbooks, scientific papers and so on are also available at the “store”) and tries to find all of the items on your list.
If he can do it, then somebody else thought of your invention before you, and your claim will be rejected. If not, then you’ll get your patent.
Now clearly, if you have a long list, it’ll be harder for the examiner to find all of the things on it in the Prior Art store. And if you have a short list like the one I proposed above, it’ll be easier for the examiner to find everything (and reject the claim).
But wait, there’s more! Let’s imagine that you got a patent for your list — the examiner couldn’t find all the items in the Prior Art store. Now, you find somebody who’s making BLT sandwiches just like your invention, and you want to stop him. You do it with the same list that the examiner wasn’t able to find in the prior art.
You take your list to the infringer’s factory, and you “go shopping” there. If you can find all of the items on your list at the factory, then you have a shot at forcing him to stop (or selling him a license, or however you want to play it).
So when you want to use your list to stop an infringer, a short list is easier / better than a long one.
To sum it up: a long list is easier to get a patent on, but harder to use effectively. A short list is harder to get a patent on, but easier to use effectively.
A good patent attorney will help you put together the right list: long enough that the examiner can’t find it all in the Prior Art (so you can get the patent); but short enough that you can find it all in a copycat’s factory (so you can stop the knock-off).