pron, det

 

information used to ask for information about something

What's this?

 

the thing used to refer to something without naming it

I heard what he said.

 

not heard used when you have not heard what someone has said and you want them to repeat it. Some people think this use is not very polite.

"Do you want a drink Tom?" "What?"

 

reply used to ask what someone wants when they call you

"Hey Jenny?" "Yes, what?"