Punctuation is the visual sign which helps a reader distinguish between words and sentences and helps the reader understand the relationships between words.
Technically, capitalizing, spaces between words, and indentations at the start of paragraphs are all forms of punctuation. But usually when we speak of punctuation, we speak of the symbols we use to mark sentences--the punctuation marks. They help the reader understand exactly what the writer's intentions are.
In English the following punctuation marks are normally used: period, question mark, exclamation point, comma, semicolon, colon, quotation marks, single quotation marks, italics, underlining, dash, hyphen, parentheses, brackets, ellipsis, and virgule.