An 'intonation unit' is a piece of utterance, a continuous stream of sounds, bounded by a fairly perceptible pause. Pausing in some sense is a way of packaging the information such that the lexical items put together in an intonation unit form certain psychological and lexic~grammatical realities. Typical examples would be the inclusion of subordinate clauses and prepositional phrases in intonation units.
It is proposed here that any feature of intonation should be analyzed and discussed against a background of this phenomenon: tonic stress placement, choke of tones and keys are applicable to almost all intonation units. Closely related with the notion of pausing is that a change of meaning may be brought about; certain pauses in a stream of speech can have significant meaning variations in the message to be conveyed. Consider the example below, in which slashes correspond to pauses (Roach, 1983:146) (see Halliday, 1967; Leech & Svartvik, 1975 for more): the meaning is given in brackets.

Those who sold quickly / made a profit
(A profit is made by those who sold quickly.)
Those who sold / quickly made a profit
(A profit was quickly made by those who sold.)

More examples can be used in order to illustrate the significance of pausing, and further, it can be pointed out that right pausing may become a necessity to understand and to be understood well.
Tonic Stress
An intonation unit almost always has one peak of stress, which is called 'tonic stress', or 'nucleus'. Because stress applies to syllables, the syllable that receives the tonic stress is called 'tonic syllable'. The term tonic stress is usually preferred to refer to this kind of stress in referring, proclaiming, and reporting utterances. Tonic stress is almost always found in a content word in utterance final position. Consider the following, in which the tonic syllable is underlined:

I'm going.
I'm going to London.
I'm going to London for a holiday.

A question does arise as to what happens to the previously tonic assigned syllables. They still get stressed, however, not as much as the tonic syllable, producing a three level stress for utterances. Then, the following is arrived at., where the tonic syllable is further capitalized:

I'm going to London for HOliday.

Tone
A unit of speech bounded by pauses has movement, of music and rhythm, associated with the pitch of voice (Roach, 1983:113). This certain pattern of voice movement is called 'tone'. A tone is a certain pattern, not an arbitrary one, because it is meaningful in discourse. By means of tones, speakers signal whether to refer, proclaim, agree, disagree, question or hesitate, or indicate completion and continuation of turn-taking, in speech.
Pointing to extensive variations in the taxonomy of English tones, Cruttenden (1986:58) rightly notes that 'This is an area where almost every analyst varies in his judgement of what constitutes a 'major difference of meaning' and hence in the number of nuclear tones which are set up.' He adds: '...intonational meanings are often so intangible and nebulous ... (that) it is difficult to see how a wholly convincing case for any one set of nuclear tones..' (parenthetical statement is mine). Crystal (1969) and Ladefoged (1982) identify four basic tones (fall, rise-fall, rise, and fail-rise) while O'Connor and Arnold (1973) distinguish only two (rise and fall). Brazil et al. (1980) and Roach (1983) endorse five tones (fall, rise, rise-fall, fall-rise, and level) whereas Cruttenden (1986) recognizes seven tones (high-fall, low-fall, high-rise, low-rise, fail-rise, rise-fall, and mid-level).
It appeared in the author's teaching experience that only four types of tones can be efficiently taught to non-native speakers of English:

fall
low-rise
high-rise
fall-rise

What makes a tone a rising or failing or any other type of tone is the direction of the pitch movement on the last stressed (tonic) syllable (Brown, 1977:45). If the tonic syllable is in non-final position, the glide continues over the rest of the syllables. A fall in pitch on the tonic syllable renders the tone as 'fall'. A 'rise' tone is one in which the tonic syllable is the start of an upward glide of pitch. This glide is of two kinds; if the upward movement is higher, then it is 'high rise'; if it is lower, then it is 'low rise'. 'Fall-rise' has first a pitch fall and then a rise.

thank you
it is good if it is outlined and summarized