Counting 'unique' strings?

I’m hoping one of the PHP/MySQL gurus can answer a question for me.

I want to know how to query a table and return the number of ‘unique’ strings contained within a column.

For example, say I have a column called ‘domains’ which contains the following:

google.com



google.com
facebook.com

There are 5 unique domains as google.com should be only counted once.

I’m a bit lost on how to do this. Any help would be great! Cheers.