Housman's poem "Wake Not for the World Heard Thunder" is 3 stanzas of octaves with 8 lines per stanza. The lines are written in trochaic tetrameter. This means that the rhythm of the lines is derived from one stressed beat followed by one unstressed beat, and the pattern carries on for 4 feet of repetition, like this (capital letters are stressed):
DA da / DA da / DA da / DA da
As an aside, we can't really say one stressed syllable followed by one unstressed syllable because (prior to Dr. Samuel Johnson's era, at any rate) poets varied the syllabic stress through elision and the pause thus making both integral to understanding and rhythm.
Elision shortens a line by eliminating syllables, as I'll illustrate in the following line:
Whene'r / she loved / th' heart / won o'er.
Silly as my line is, elision in the line shortens the 3 syllables of "whenever" to 2 with "whene'r." "The" and "over" are similarly shortened, so the line is iambic (da DA) tetrameter.
The pause has traditionally been used to the opposite effect. It is derived from Old English poetry's caesura and may fill in unstressed beats in lines. I'll again illustrate:
When, / love is / past the / tide, / Hope may / spring --.
In an equally silly line, the commas (and em dash) represent pauses that are integral to the meaning of the line and fill in unstressed beats in the rhythm, which is trochaic hexameter.
In Housman's poem, there are a number of catalectic, or truncated, lines that reduce the last metrical foot by an unstressed beat, thus altering and varying the rhythm. Here is an example that is scanned (' is a stress mark):
Fear' not / you' for / flesh' or / soul';
The final unstressed beat is eliminated thus truncating the line. Compare this to a full line:
And' the / fife' with / death' -notes / fill' -ing
The rhyme scheme that accompanies the tri-octave trochaic tetrameter poem is:
a b c c a b d d
e f g g e f h h
i j k k i j l l