The rules about how to cite are based on what you're citing. Some things are the same no matter what you cite, but some things change. The basic things you might cite are:
Most "other stuff" and many web sites will mean you have to look at a copy of the MLA handbook to see all of the ways you can handle weird citations. This Guide will only address citing the more common and easy citations.
Books include hardcopy, print books as well as online books. Books have been around the longest, and they're the easiest things to cite. This style is for when you're using the whole book, not just a little part of it.
Articles in periodicals - repeat publications like magazines, newspapers, and journals - are a little more complicated. Whether you read the magazine/newspaper in hardcopy or online, articles work almost the same as long as you know that it's an exact copy of the original. If you're not sure if it's the original - like a newspaper's editorial blogs for example - cite it like a web page.
This includes a chapter in a book, an article in an encyclopedia, an essay or short story in a collection, or any other section of a book where you only used that one section and nothing else in the book.
Web sites that are just digital books or articles are cited almost the same as print books and articles. But when you have a web-original like an organization's website, a government statistic, or other "stuff" on the web, it gets more complicated. We'll give you some ideas, but when in doubt look at the MLA Handbook for the whole story!