By looking at each chord triad individually, and in particular at the intervals in each chord, it is possible to find out what kind of chord, major/minor etc, each one is.

Lets look at the first chord built on C:

From the root C of the chord, the intervals are major 3rd and perfect 5th. From the Intervals section in The Basics you can see that this combination of intervals produces a major chord.

Now lets have a look at chord 2.

From the root D of the chord, the intervals are minor 3rd and perfect 5th. From the Intervals section you can see that this combination of intervals produces a minor chord.

You may wonder why in a major key we have a minor chord. To make the Dm chord into a D major chord the minor 3rd interval (3 semitones) would have to become a major 3rd interval (4 semitones).

The only way to do this is to raise the pitch of the F by a semitone which produces the note F#. However, because F# is not part of the key of C it does not belong in any chord in the key of C. So the F must remain as an F and therefore the D chord must remain Dm.