A quotation mark or "inverted comma" is one of the punctuation marks placed at the beginning and end of a word, phrase, sentence, or passage to denote that the enclosed text is a quotation. (i.e. they are the words of a secondary source rather than of the author of the present text.)
The American convention is to use double quotation marks for primary level quotations, and single quotation marks for secondary level nested quotations, where the quoted text itself contains a quotation. For example:
Tertiary, quaternary, quinary, etc. levels of quotations alternate double and single mark usage. The British convention is the opposite, with the primary quotation using single marks, the secondary double, etc.
Traditionally, opening quotation marks differ from closing marks in their orientation, typically being rotated 180°:
However, with the spread of more simplified computer-based typographical conventions, uniform quotation marks (") have become popular, especially on the Internet.
Quotation marks are sometimes used on words or short phrases (but not typically sentences or longer passages) to denote the questionability of the word or phrase due the assumptions upon which it is used, the questionability its literal meaning, or to denote irony. When used in this manner, the marks are often referred to as "scare quotes". While widespread, this usage is considered controversial, as often the marked text isn't a direct quotation. One theory behind this usage is that the word or phrase being thus marked is one that would be used by a hypothetical person taking the position the writer is criticizing, even if no actual person has used that word or phrase.
The angled quotation symbol ` is not used in formal grammar, but is used in some forms of computer language, notably within UNIX shell-scripting.