Major or minor chords,as you probably know are called triads (triads = chords which are built from 2 thirds or, in other words, 3 notes)
there are 4 types of triads we use
major (major third + minor third ,or 2 tones +1.5 tones)
minor (minor third + majord third, or 1.5 tones + 2 tones)
augmented (two major thirds)
dimished (two minor thirds)
if we use the notes of the major scale C, we can get the following triads: (using the notes of the scale only)

C=major c,e,g (c to e =2 tones, e to g =1.5, major chord)
D=minor d,f,a (d to f = 1.5 tones, f-a = 2 tones , minor chord)
E= minor
D= minor
F=major
G=major
B=diminshed (two minor thirds: from b to d theres only 1.5 tones and from d to f there are 1.5 tones as well)

the same formula can be applied to any major scale.
in minor and harmonic minor scales, the chords appear in different order but the idea remains the same.

7th chords
7th chord is basicly a triad with another third added to it.
there are 7 types of 7th chords (should be 8 but since the aug7 is the octave, the augmented7h chord - aug. triad + aug.7 does not exist).
using the same formula ive used before to find out about the triads, we can see that the 7th chords which will suit the C major scale
are:
C= major 7th (major triad + major 7th)
D= minor 7th (minor triad + minor 7th)
E = minor 7th
F = major 7th
G= dominant 7th (this chord is "special" in harmony terms , if you want any further explanation just ask :)
major triad + minor 7th
A = Minor 7th
B= Half diminished (dim triad + minor 7th)
sometimes B can be Diminished 7th as well (dim triad + dim 7th).
as you probably noticed there are only 5 types of 7th chord out of 7 ,i have mentioned.
the other 2 are :
minor triad + major 7th -it iss a chord which will usually be solved into minor chord (for example Cmin7+ will be solved into Cmin)
this chord can be used as the first Degree of the minor scales (by minor scales i mean the natural minor, harmonic minor and the melodic minor )(in Cmin scale it can be the chord of the note C )

augmented triad + major7th will be usuaully used as the chord based on the third note of the harmonic and the melodic minor scales.



I hope that makes sense.
if it doesnt , you can always ask me again :)