The possessive indicates that one noun takes ownership of another noun. There are two forms of showing possession: using an apostrophe or the word “of”:

• That is Ben‘s dog. (singular possessor, so “s” follows the apostrophe)
• We are waiting for the doctors’ input before making a decision. (plural possessor, so the “s” is followed by the apostrophe)
• What is the opinion of the group?
• Tony is a cousin of mine.

Of the two methods used to convey possession in English, the apostrophe is generally used more often.