A more thorough explanation:

Sic (pronounced "sick") is a Latin word that means "so" or "thus." In writing, it is used to show that a word or phrase in a quote is spelled or used exactly as it appeared in the original document. It is always enclosed in brackets and usually set in italics.

Example 1: "The article stated that the company was 'very sucessful [sic].' "

Example 2: "The sign read 'No Trespassing [sic],' indicating that the spelling error was in the original sign."

In Example 1, the word "sucessful" is misspelled in the original article, but the writer is quoting it exactly as it appeared. The use of [sic] indicates that the writer is aware of the error but is reproducing it exactly as it was written.

In Example 2, the sign has a spelling error in the original, and the use of [sic] indicates that the writer is aware of the error but is quoting it exactly as it appeared on the sign.