A trade secret is any piece of information used in a business that isn't generally known to the public. This is valuable because the information is kept secret. Trade secret law can apply to a broad variety of information, including formulas, patterns, business plans, designs and procedures. The law provides some protection against others from misappropriating, or improperly obtaining, your secret.
People rely on trade secret law because of the shortcomings of other areas of law.
Copyright law prevents others from copying certain works, but copyright law only applies to the expression of ideas and not the underlying idea itself. Patent law prevents others from using your idea, but some things, such as recipes, aren't patentable. Furthermore, patents only last for a specific period of time. It's possible to keep a trade secret forever. And the secret formula can't be patented because it would be considered a recipe.