In a
suspended triad, the
third has been replaced by another note. This is either the 2nd or the 4th. So, if we take a C
major triad:
C, E, G, and replace the E (third note of the C
major scale) by a D (second note of the C major scale), we get
C, D, G - a Csus2 triad. If we replace the E with an F, we get
C, F, G - a Csus4 triad.
Note that if we took the C minor triad, C, Eb, G and replaced the Eb (which is the minor third) with D or F, we would get the exact same suspended triads. This is the reason that suspended triads are neither major or minor - the flavour of the chord (major / minor) is determined by the third.