(rhythm, intonation and stress-timing)
The languages of the world may be classified in many ways. One distinctive classification is how the timing of tone groups are arranged. Tone groups in speech are bursts of sound, or sound frames, which contain a chunk of coded language patterns. A tone group is said in a single breath. These chunks seem to be rather similar to the 'packets' of information which are sent through packet-switching networks on the internet, and their size is closely related to the needs of our human processor, the brain.
There are various options for arranging the sounds in tone groups. We usually recognize these choices as rhythm, intonation and stress; (actually intonation contains a large number of more detailed features). Stress is essentially anything which marks one bit of sound out from the surrounding speech stream. It is typically made up from a subtle combination of duration, speed, pitch and loudness.
The feature of sound duration is often called timing. The timing method of sound chunks varies is in a continuum amongst languages. At one extreme of this continuum are so-called stress-timed languages, and the at the other extreme are so-called syllable-timed languages. In practice, no language is entirely syllable-timed or entirely stress-timed.
In syllable-timing, each syllable has the same time duration. This means that tone groups vary in duration, depending upon the number of syllables they contain. Because a tone group is said in a single breath, in practice this variation in tone group length is limited. Thus in a tone group with more syllables than usual, all the syllables might be said more quickly to 'fit within a single breath'.
In stress-timing, each tone group has more or less the same time-duration, (a single breath) regardless of the number of syllables it contains. This means that some syllables will be spoken very quickly, while the stressed syllable or syllables will often have a much longer time duration. If the tone group has an unusual number of syllables, everything might be speeded up, but stressed syllables will usually take relatively longer to say than unstressed syllables. In English, tone groups average about five syllables (though it is possible to have a tone group of only one syllable !).
Usually (but not always) the stressed syllable in a stress-timed language is the one containing new information. By changing the time taken to say any particular syllable in a stress-timed language, the meaning of that tone group can be changed. This is a very tricky game indeed ! Native speakers do it automatically, but the speaker of a syllable-timed language who tries to learn a stress-timed language will probably have great trouble mastering the new arrangement (and meanings) of sound patterns.
English is very strongly a stress-timed language. Chinese is strongly syllable-timed. Korean is more or less syllable-timed. Japanese is timed by 'mora'. (A mora can be C+V (consonant + vowel), V, C+/y/+V, the sound /n/, or a special voiceless pause between certain consonants. Each mora has the same duration).