What is the difference between "utter" and "speak"?

E.g.

• speak vehemently
• utter vehemently

When should one use speak and when utter?

Solution 1:

It's useful to think of it this way: To utter means to make a sound; to speak means to make an intelligible sound.

I'm being deliberately simplistic here just to underscore the main difference. The truth is a little more complicated.

NOAD:

utter 2 verb [ trans. ] 1 make (a sound) with one's voice : he uttered an exasperated snort.
• say (something) aloud : they are busily scribbling down every word she utters.

So utter also means to "say aloud"; but speak has a more precise definition. (And one wonders why "aloud" is needed at all in the above entry.)

speak |spēk| verb ( past spoke |spōk|; past part. spoken |ˈspōkən|) [ intrans. ]
1 say something in order to convey information, an opinion, or a feeling : in his agitation he was unable to speak | she refused to speak about the incident.

Not that this is always the case. Every word you speak aloud is an utterance, but not every utterance is of informational value.