One thing I often find myself correcting when proofreading is the misplaced apostrophe. But while apostrophes seem to be the source of much confusion, they're actually simple to use.
Apostrophes are used to indicate possession:
If something belongs to one person, the apostrophe goes before the s, as in the employee's rights.
If something belongs to more than one person, the apostrophe goes after the s, as in the employees' rights.
The exception to this is the word it:
Its is used to indicate possession, as in the investment has shown its value.
It's, on the other hand, is a contraction of it is, as in it's here.
This brings us to their other use … to represent missing letters, as in I'm, they're, wouldn't and so on.
One final point … apostrophes are never used to make a plural unless adding an s on its own would cause confusion, such as when an abbreviation or single letter is made plural (as in it has two f's).