When quoting material from sources in your writing, you should generally try to trim down the quotes as much as possible. Sometimes though, for one reason or another, you may need to quote a long (generally a hundred words or more) passage from something. In these cases, you don’t integrate the quote into your writing as you would with smaller excerpts, you need to use a block quote.
According to the Chicago Manual of Style:
In typeset form, quotations may be either run in — that is, integrated into the text in the same type size as the text and enclosed in quotation marks — or set off from the text as block quotations, or extracts. Block quotations are not enclosed in quotation marks and always start a new line. They may be indented or set in smaller type or a different font from the text; they may have unjustified right-hand margins or less space between lines. These matters are normally decided by the publisher’s designer or by journal style. Authors preparing block quotations should avoid such devices (unless otherwise advised by their publishers) and simply use the indentation feature of their word processors.
In reality, block quotations should be a rare thing and, as the CMS notes, the rules for how and when to use them will usually be given by the instructor, editor, or publisher. When in doubt, ask.