"Its" is the possessive of the pronoun "it." Note that there is no apostrophe. Example: Its appearance was misleading.
"It's" is a contraction meaning "it is." Example: It's a long way to Tipperary. "It's" can also be a contraction meaning "it has." Example: It's been a long day.
"'Tis" is also a contraction meaning "it is." Example: 'Tis seldom used in modern English.
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