Humanities Documentation page: http://bcs.bedfordstmartins.com/resdoc5e/RES5e_ch04_o.html
Each academic discipline uses a style manual so that source (bibliographic) information is conveyed in a predictable and standardized way, making it easy to follow up interesting references, to make sure material has been used appropriately, and to give credit to the folks who originally did the work. That is, citations function as a kind of scholarly generosity and courtesy, as a means of covering your bases, and as respect for other peoples' work (i.e. prevention of intellectual theft a.k.a. plagiarism -- see the next section of these resources).
Always take note of the source of what you want to cite, along with the page number. (This is why citation management tools are incredibly useful.) I speak here from bitter personal experience. Often a reference librarian can find the obscure tome that the chapter or article is in, but all that work could be saved by taking good notes.
Even with all the apps for building databases of references and citing them, you have to know the parameters of the style manual you've chosen, because they don't always generate properly formatted citations -- and "Zotero formatted it wrong" is right up there with "the dog ate my paper" as an excuse.
Just a heads up: you can't just copy and paste library bibliographical records into your bibliography -- the order of elements, punctuation, and spacing are often quite different from most standard citation modes. You *can* of course try out the citation generators on the web and within the websites and databases that you use, but, again, it's also good to know the format for the style manual you're using, because they don't always generate exactly what you need.