Prosodic (or suprasegmental, they're the same thing) features are those elements of a language's phonology that cannot be expressed by describing the articulation; in other words, prosodic features are pitch, stress, and length. I will explain each feature in better detail below.

Most languages in the world are pitch languages; English, believe it or not, is in the minority. What, exactly, is a pitch language? Any language that uses the pitch of a phoneme to differentiate it from other phonemes is a pitch language; the classic example is chinese, which uses five tones. Pitch allows a language to conserve syllables; why agglutinate, like Japanese or Quechua, and create monstrous words when the same phoneme can express the four concepts of mother, horse, hemp, and swearing? (These are, using pinyin and the numeric pitch representation, ma1, ma2, ma3, and ma4.) In the case of pitch, regular articulatory phonology does not describe the frequency at which the vocal cords vibrate, only whether they are vibrating (voiced consonants and vowels) or letting air through (stops). Seeing as it is a major feature of a language, it must be included under prosodics.

Another aspect of pitch must be mentioned: the pitch contour. English may not differentiate between different syllables by pitch, but speakers use the overall pitch of a sentence to imply meaning. What do you get when you raise the pitch of your voice at the end of a statement? A question.

Stress is an overlooked feature of English; it does not play as large a role as in other languages (see Russian Vowel Reduction for a look at the stress system of Russian), but placing emphasis on the wrong syllable will get you strange looks. Stress is a very simple device: it is basically pronouncing a segment of a word more loudly, and, usually, lengthening the vowel. Russian changes regularly depending on where the stress is placed in a word. English is much more complex; try pronouncing your favorite words with the stress in different places to see what I mean.

Related to stress is length; neither are described in articulatory phonetics, as the description "low, front" does not include how long to keep the airstream flowing through your mouth and nose. Many starting linguists mistake the difference between vowels of short and long length as the same between tense and lax. These are NOT the same; the difference between the vowel sounds of "foot" and "boot" is NOT length, but how tense the muscles in your throat are. I cannot think of any examples of varying vowel length in English, but I do know that Estonian, Japanese, and, I believe, Finnish, have different lengths of vowels. In Japanese, Tsuuji is a proper name; tsuji is to move one's bowels. This mistake in pronunciation has been made before, with much embarassment.