There is much confusion about the proper use of the apostrophe. There is even a movement to get rid of it in some countries. The apostrophe is correctly used to show possession, as part of a contraction to replace missing letters, and to pluralize in odd cases such as pluralizing a lower case letter. An example of this pluralization would be, p's rather than ps, use here seems to be more for clarity. It is not used to create plurals of nouns, nor to indicate a decade such as the 1960s (inaccurate version 1960's).
Here are some examples of common misuse and the correct usage:
There are many more examples of confusion when using this punctuation mark. Hopefully the examples here will help clear up some of the common errors.
Some parts of English grammar can be very confusing. One common example is the difference between "it's" a contraction of "it is" and "its" a possessive pronoun. This page contains a grammar tip: "it's" vs "its".