The difference between 'clear data' and 'clear cache' on a mobile phone

[Muzz]

I have an HTC phone. I've been receiving messages recently that tell me my internal storage is low. I use the phone for texts, phoning people and the Internet. I usually look at AB (!), news, Facebook and sometimes check e-mails.
I do try to 'clear history' on the Internet.
But, when I looked at Settings, I wasn't sure what to click on when it came to 'clearing out'.

For example, on Mail, it shows Total 4.09MB, Application 3.97MB, Data 120KB. There is the option to 'clear data' but the 'clear cache' is greyed out.

On Facebook, it shows, Total 18.41MB, Application 5.56MB, Data 12.85 MB. Again, there is the option to 'clear data' but not to 'clear cache'.

IF I click on 'clear data', what sort of data does it clear?

Thanks for any help! Muzz