The C major scale is made up of all the white notes on a piano keyboard, but other major scales (starting on different notes) include black notes - G major, for example, contains one sharp note, F#, ...

A scale is a sequence of notes that are played in a mostly step-wise motion, over a duration of an octave (e.g. from C to C).

For instance, in a C major scale, C=1, D=2, E=3 and so on, until B=7. So, to build a C major triad, we need the notes numbered 1, 3 and 5 from the major scale; in other words, C, E, and G = C major.