Contractions are used in everyday speaking and in informal writing. A contraction is a shortened version of two or more words, generally formed by placing an apostrophe in the place of any deleted letters.
A commonly used contraction is "it's," made from the two words "it" and "is." The apostrophe replaces the "i" in the word "is." Some contractions have irregular construction. "Won't" is the contraction for "will not."
In formal writing, writers generally avoid the use of contractions, spelling out the words instead. Formal writing includes school essays and papers, business letters and resumes. A frequently accepted exception to this rule is with the word "o'clock," which is the contraction for "of the clock."