The emphasis given to one or more words in a sentence is:

volume.
stress.
pitch.
tone of voice.

http://dictionary.reference.com/browse/stress

The emphasis given to one or more words in a sentence is called stress. Stress is the relative prominence or loudness of a syllable or word in speech. It can be used to highlight important information or convey meanings and emotions. Stress can be achieved by varying different aspects of speech, including volume, pitch, and tone of voice.