Trade secret is a very broad concept. It could include inventions but it does not have to. It could be any confidential information that is maintained by a company and not disclosed to the whole world or an entity. Trade secrets are valid intellectual property protection as long as they remain a secret. However, as long as a company keeps a secret and takes steps to maintain that secret so that not too many people know about it, well it remains indefinite because nobody ever discovers it!
Patents, on the other hand are not indefinite. Patents are the definite acknowledgment of inventions and they protect certain kinds of use. You could protect certain kinds of trade secrets that are inventions through patents but that would require you to make that information public because a patent is a disclosure of your idea in which you explain how to make and use your invention and disclose what portion is new and what portion you are claiming your limited monopoly right to. Plus, you only get that monopoly right for twenty years.
A patent is a limited monopoly right because it has a limited duration, whereas trade secrets can go on forever. Therefore, trade secrets can be much broader and are not limited by things that are actually inventions. They might include other things that are ideas, works of authorships as long as there remains a secret to it. It might surprise you to learn that there are certain very famous things that are protected by trade secrets that were never patented. For example the formula for Coca-Cola and the recipe for Kentucky Fried Chicken – these things could be potentially protected by patents if they are new, inventive and not obvious but they are instead maintained as trade secrets. You could probably guess why – Coca-Cola’s formula and the Colonel’s secret recipe could only be patent protected for a number of years, whereas keeping them secret could potentially protect them forever.
The other thing to understand with trade secrets versus patents is that sometimes it is easy to discover the trade secret simply by looking at the end product. For instance, if I look at something and I can tell exactly what goes inside it (and the real invention is the stuff that goes inside it) the trade secret may be easily discoverable by others as well. Consider Coca-Cola: if it were in fact possible to discover exactly with 100% certainty that a particular chemical composition is Coca-Cola’s formula, well then, it would be better for the company to file a patent because it is easier to detect the infringement when the cat is in fact out of the bag. With that patent, you can securely enjoy your monopoly right, whereas if you kept the formula as a trade secret and someone figured out the ingredients from the end product and then created and marketed it on their own, it would be hard for you to stop them because they did nothing wrong. They just got their information from independent discovery and observation. Of course you can take active steps to stop the theft of trade secrets from within your organization or company. You do want to make sure a trade secret are protected so that there are not too many people who know about it. For example, if you are an independent inventor, you might also create a log of that trade secret, put it in a safety deposit box, keep a key or put it in the registered federal government office so you have a documented proof of when that invention or trade secret was made. It is very important you do not release that information. The information that is in that envelope, for example, needs to be secure and maintained as a secret. As long as affirmative steps are taken to keep things a secret, then it's going to be easier to enforce the trade secret rights if the secret is breached by an employee or stolen from someone else. The difference, though, with the patent is primarily that you must disclose to the whole world the details of your invention. Patents are issued only for inventions, whereas trade secrets can go to any kind of confidential information.