Punctuation, from the Latin punctus (point), the use of marks to separate words from each other in sentences and parts of sentences. The Western system of punctuation arose in the 9th century a.d. when periods, commas, colons, and semicolons first came into general use. The following are the punctuation marks and some of their functions in the English language:
The comma (,) is the basic unit of punctuation, serving to clarify the structure of a sentence by separating words, phrases, and clauses.
The period (.) marks the end of a completed sentence. It is also employed in abbreviations, as in J. Smith, i.e., and q.v. In mathematics it separates whole numbers from decimal fractions.
The semicolon (;) is a separating mark of greater emphasis than the comma and appears chiefly in long sentences. It may be used to separate a succession of phrases or entire sentences when they are in some way related to each other, as when they constitute a series of examples.
The colon (:) is used after an introduction and before an explanation, example, or statement, as in "This was his only excuse: he was very young." It is also used to separate hours from minutes, as in 2:15.
The hyphen (-) is a mark employed to divide a word that carries into the next line and to connect a compounded word, such as well-being.
The dash (—) serves many purposes, the most conventional being its use where there is an abrupt change in statement, as in "Thus the plot thickens—but I'm boring you."
Parentheses () enclose a word or phrase inserted in a sentence which would be complete without it. They may also enclose a complete sentence which is deemed supplementary to the paragraph in which it appears. Serving a similar purpose, brackets  are often used when the insertion is supplied by the editor instead of the author.
The apostrophe (') marks the possessive case, as in John's book or James' cat, and is also used to indicate the omission of letters, as in I'll for I will, and the clipping of words in dialect, as in nuthin' for nothing.
Quotation marks ("") may enclose direct quotations, unconventional words, aphorisms, and other possible types of expression. Single quotation marks (") are sometimes used instead of double marks, but most often appear when quotation marks are called for within a passage which is itself a quotation, for example: George told me "I had trouble getting her to say 'I do.' "
The ellipsis (...) serves to mark the omission of one or more words in a quoted passage.
The question mark (?) ends any sentence cast in the form of a question which requires an answer.
The exclamation point (!) ends a word or phrase intended to express great surprise or emotion.