Punctuation marks are symbols used in writing to indicate the timing and intonation in how the text is to be read.
Punctuation marks are different from symbols like the ampersand & and the hash # sign, which have distinct meanings and are actually pronounced as words when the text is read out.
They are also different from smileys or emoticons like :) which indicate the attitude in which the statement is given.
(However, sometimes the term 'punctuation mark' is used to apply to all non-alphanumeric symbols.
(The most common and likely most-used punctuation is the space. But it isn't something that is written down, so it probably can't be properly called a punctuation
Marks for timing:
Some punctuation marks indicate where pauses and breaks should go. Into this class go the space, the comma, the period when used as a full stop, the semicolon, and the dash.
Marks for intonation:
These marks indicate the tone of voice in which the sentence is to be read. These punctuation marks include the exclamation mark for loudness, the question mark for rising-questioning tone, the parentheses for asides (although parentheses are borderline between timing and intonation), and the ellipsis when used to indicate a thought trailing off...
Information on how to use these marks is at the punctuation node.