When discussing the basic, or fundamental, components of English poetry, we must start with the rhythmic nature of the language. As an illustration, compare English to French. French is an unaccented language: every syllable of a multisyllabic word carries the same weight and has the same emphasis. This is not true of English: every multisyllabic word has one syllable that carries greater weight and receives the accent of the word (el' -e -phant, a -pos' -tro -phe, en -joy', jing' -le). In addition, the nature of English very often causes this accent to fall on the second syllable thus making English compatible with iambs (/ ~ * /): be -fore', with -all', de -pend', up -right', up -on', etc. As a result of the nature of the language, poetry in English depends heavily on rhythm. Free-form poetry has long been accepted, but the best free-form poetry is conscious of rhythm, though it may not adhere to meter.
Meter is the combination of deliberate rhythm(s) plus length of line. For instance, a poet may choose a deliberate rhythm of anapests expressed in four repetitions for a line length of tetrameter. A scanned line written in anapestic tetrameter would look like this (~ unstressed, * stressed): / ~ ~ * / ~ ~ * / ~ ~ * / ~ ~ * /. Conversely, a poet writing in free form, may choose not to honor traditional metrical structure, but rhythm must be present to separate the poetry from prose.
Prose is writing that avoids rhythm. This is unless a writer decides a prose passage will be enhanced by the deliberate choice of specific word schemes, just as Martin Luther King's prose was so spectacularly enhanced by his choice of a repetition scheme called anaphora. To reiterate: poetry must have rhythm (though not necessarily meter); prose must not have rhythm, unless by deliberate choice for a momentary deliberate effect.
Rhythm and syllabic accent (i.e., emphasis) comprise the important component of timing in English poetry. Timing, consisting of rhythm and accent together, is the difference between writing lines that count syllables and lines that express ideas rhythmically. Very evident in Shakespeare's writing, a rhythmic line may have syllables elided together (e.g., wi'_th' [with the]); may have beats dropped from the beginning or end of a line forming catalectic lines; may have words with ellipsis in accord with rhetorical word schemes; and may use pause (i.e., silence) to fill the space of an unstressed beat.
The pause is an age-old component of English poetry, with its roots in the caesura of ancient alliterative poems like Beowulf. Again, Shakespeare employed pauses to create time, though less often than he used elision (drop letters) and ellipsis (drop words).
In sum, poetry differs from prose by the quality of rhythm: the former has it (poetry), the latter has not (prose). Free-form poetry may be without meter and without end-word rhyme, but it must have rhythm or, without rhythm, it is prose, even with lines written in odd lengths: the shaping of English prose lines will never make them English poetry lines. English poetry has always incorporated the pause (shown now with punctuation [, ; --]) and does so today where a pause will enhance the meaning and build the rhythmic timing of the poem, very much like a pause enhances the meaning and rhythmic timing of a song, which has a beat, say 4/4, and rhythmic timing, say highlighting triple eighth notes.