The number of times the vocal cords open and close per second is called the frequency of vibration, and is expressed in cps (cycles per second). Variations in the frequency of vibration are heard by the listener as variations of pitch (Du: toonhoogte): the more frequently the vocal cords open and close, the higher the pitch.
At the same time, pitch is used to signal the important stresses in an utterance: the position of the up or down movement of the pitch determines where we hear the stressed syllable. If you count óne-two-three-four, óne-two-three-four, etc., with the stress on one, then that syllable will be higher than the others; if you make two the high syllable, then you will hear two as the stressed syllable, etc. You can also hear the stress on a syllable because the pitch goes down for it: if you count with a question intonation (one-twó-three-four? one-twó-three-four?, etc.) you will find that you pronounce the stressed syllable (in this case two) with low pitch and that each of the ones that follow is higher than the one before, so three higher than two, four higher than three. Try it.
So, very simplistically, you might say that whether the pitch goes up or down is a matter of intonation, but that where it goes up or down is matter of stress.