A standard major chord can be composed of the root of the major scale, the third of the major scale, and the fifth of the major scale. In the key of A, for example, those notes are A, C#, and E. A standard minor chord would be composed of the root of the minor scale, the third of the minor scale, and the fifth of the minor scale. In the key of A minor those notes are A, C, and E. The use of C# or C (the major third or minor third) gives each chord their distinctive major or minor sound. A suspended chord would omit the major or minor third, but add instead the fourth of the scale or the second of the scale, thus producing a chord that does not have a distinctive major or minor tonality. These chords in the examples above would be A, D, E and A, B, E.
A suspended chord is composed of three notes where these notes have one of the following two arrangements: 1) there are five semitones between the first and the second note and two semitones between the second and the third note; or 2) there are two semitones between the first and the second note and five semitones between the second and the third note.
"sus4" or "sus2" can be used to denote a suspended chord with a perfect fourth or a suspended chord with a major second respectively. For example, Asus4 is the chord A, D, E and Asus2 is the chord A, B, E. Seventh suspended chords also exist. For example, the A7sus4 would be A, D, E, G.