“Who's" is a contraction for “who is" or “who has." For example:

Who's knocking at the door?

“Whose," on the other hand, is the possessive form of “who." For example:

Whose partner forgot to call?

These two words are easy to mix up because we usually indicate possession by adding an apostrophe and the letter “s." For example: “Don't play in the dog's water bowl." However, you can't apply that same rule to “who."

When sorting out which to use, an easy test is to try out the long form of the contraction to see if it works. If you can substitute “who is" or “who has" and still have a meaningful sentence, you want to use “who's:"

Who is eating my cheese?

That works, so you can safely use “who's:"

Who's eating my cheese?

On the other hand this doesn't work:

Who is car is blocking mine?

So, you'll want to use “whose:"

Whose car is blocking mine?

A similar problem is the choice between “it's" and “its." The same case applies here. “It's" is a contraction for “it is" or “it has," as in:

It's my turn!

“Its" is a possessive form, as in:

The car won't lose its momentum as it goes downhill.

The same trick can be used to determine whether to use “it's" or “its." Read the sentence using “it is" or “it has." If it works, then you should use “it's." If not, you probably need “its."

Some writers wonder if that apostrophe is really that big of a deal. It is. Knowing when and how to use it can make a difference in meaning and in the reader's perception of your abilities as a writer.