The difference between Come again and What

Come again is also verb with the meaning: could you repeat that? repeat that please.

What is also determiner with the meaning: which.

What is also noun with the meaning: something.

What is also pronoun with the meaning: which thing, event, circumstance, etc.:.

What is also adverb with the meaning: in some manner or degree.

check bellow for the other definitions of Come again and What

Come again as a verb (idiomatic, informal):

Could you repeat that? Repeat that please.

What as a pronoun (interrogative):

Which thing, event, circumstance, etc.:
What as a pronoun (relative, nonstandard):

That; which; who.
What as a pronoun:

That which; those that; the thing that.

Examples:

"He knows what he wants."

"'What goes up must come down."

"Keep up with what your friends are doing."
What as a pronoun:

Whatever.

Examples:

"I will do what I can to help you."

What as an adverb (usually followed by "with"):

In some manner or degree; in part; partly. See also what with

Examples:

"'What [[with]] [[singing]] [[and]] [[joking]], [[the]] [[time]] [[passed]] [[quickly]]."
What as an adverb:

Such.

Examples:

"'What [[a]] [[pity]]."

"'What [[a]] [[beautiful]] [[day]]!"
What as an adverb (obsolete):

Why?
What as an adverb (now, rare):

Used to introduce each of two coordinate phrases or concepts; both…and.
What as an adverb (Singlish):

What as a noun (obsolete, uncountable):

Something; thing; stuff.
What as a noun (countable):

The identity of a thing, as an answer to a question of what.
What as a noun (countable):

Something that is addressed by , as opposed to a person, addressed by .