Referencing styles are referred to by the institutions where they were first or were most commonly used. For example, one of the in-text referencing styles is called the Harvard style. The footnote style is called the Chicago style. Each has advantages and disadvantages. For example, the Harvard style is easier for the writer, but it is imprecise and if a number of authors and their works are cited in one set of brackets, it interrupts the flow of the text for the reader. The footnote style requires more work for the writer but is precise, specific and, by being placed at the bottom of the page, more convenient for the reader.
A footnote gives precise information. If the source is a book, it gives information in a line with each part separated by a comma or semicolon. First you must write the full name of the author or authors, then the full title in italics. Next is the city in which it was published, the publisher and then the page which holds the words that are referred to. If the source is a journal or periodical, the title is not in italics but the name of the journal is. This is followed by the issue and volume numbers, the publisher and the page number.
Footnotes should be used when quoting verbatim a sentence, phrase or a number of sentences from another author. They should also be used when you paraphrase another person's work. If you are quoting any statistic or set of statistics or information that is not "common knowledge," you should reference it with a footnote. For example, if you were writing about the demographics of the United States, the approximate population -- 300 million -- can be held as common knowledge. But if you were giving information on ethnic origins, gender, ages or state statistics, you would be expected to reference the source of the information.
If you are using an idea or theory that is generally credited to one person or a group of people, you should reference it whether you actually use any of their words or not. Footnotes can also be used for giving additional information which is too detailed for the main text but which some readers may find useful and may wish to know.