If a noun names a specific person or place, or a particular event or group, it is called a proper noun and is always capitalized. Some examples are Eleanor Roosevelt, Niagara Falls, Dracula, the Federal Bureau of Investigation, the Great Depression.
Unfortunately, some writers assign proper-noun status indiscriminately to words, sprinkling capital letters freely throughout their writing. For example, the Manhattan Project is correctly capitalized because it is a historic project, the name given to the specific wartime effort to design and build the first nuclear weapons. But the common noun project should not be capitalized when referring to a club's project to clean up the campus, for example. Similarly, the Great Depression should be capitalized because it refers to the specific period of economic failure that began with the stock market collapse in 1929. When the word depression refers to other economic hard times, however, it is not a proper noun; it is a common noun and should not be capitalized.
Some flexibility in capitalizing nouns is acceptable. A writer may have a valid reason for capitalizing a particular term. For example, some companies have style guides that dictate capital letters for job titles such as manager. But often the capitalization beyond the basic guidelines is an effort to give a word an air of importance, and you should avoid it.