Phonetics and the IPA 3, Exercise 3: minimal differences

[p] and [m]
  Shared: both are bilabial, made with a full closure of the lips.
  Different: [p] is a voiceless oral stop (velum raised, no air through the
  nose); [m] is a voiced nasal (velum lowered, air through the nose, vocal
  folds vibrating).

[s] and [t]
  Shared: both are voiceless and alveolar (tongue tip or blade at the ridge).
  Different: [t] is a stop (the airway is fully blocked, then released);
  [s] is a fricative (a narrow gap stays open, giving continuous hissing
  turbulent air).

Takeaway: changing exactly one of voicing, place or manner gives a
neighbouring sound; that is why the chart is a grid.