If you use the tones of the major scale to create triads on each scale degree, you will get three types of chords: major, minor and diminished. The Roman numerals indicate the scale degree from one to seven with uppercase depicting major, lowercase depicting minor and ° depicting diminished chords.
If you go one step further and place a seventh on top of these triads, you will get four types of chords: major seventh, minor seventh, dominant seventh and minor seventh with a flatted fifth (m7♭5), also called half-diminished seventh (ø7). The resulting diatontic seventh chords are ordered as follows:
These chords are make up most jazz, funk, gospel and soul music. Of course there is always more to the story, but seventh chords are the essential and necessary ingredient of these styles. They play a very important role in blues as well. So knowing the different appearances of seventh chords is an essential skill for everyone who wants to play in these styles.
For example the ii-V-I progression, the most common chord progression in jazz features three of these seventh chords, the minor seventh, dominant seventh and major seventh chords on scale degrees two, five and one.
There are other seventh chords in addition to the four ones mentioned above. One that is used a lot in jazz is the diminished seventh chord. The minor major seventh and augmented major seventh chords create very special sounds, but are not used that frequently.
Check out the Piano Chord Charts book for seventh chords in all keys.