In general, to form a possessive of a singular noun (regardless of whether it ends in s) or a plural noun that doesn't end in s, add 's to the end of the word. For a plural noun that does end in s, add an apostrophe but no additional s.
Examples of correct possessives
- user's (singular); users' (plural)
- Elise Williams's
- the Williamses'
Examples of incorrect possessives
Exception: The possessive of it is a special case because it doesn't take an apostrophe.
Recommended as possessive: Each component has its own set of traits.
However, the contraction of "it is" takes an apostrophe.
Recommended as contraction: It's preferable when accuracy isn't as important.