Seventh chords can be analyzed using a two-word description, where the first word corresponds to the size of the triad (the first three notes in the chord), and the second word corresponds to the size of the interval between the bottom note and the top note (which will always be a seventh of some sort, called the 'outside' seventh).
An example: Major-minor is probably the most common type of seventh chord; the dominant seventh chord in a major scale is Major-minor (abbreviated M-m; a capital M stand for Major, and a lowercase m stands for minor). In C Major, the dominant seventh chord is G-B-D-F. The bottom triad, G-B-D, is major (see triads for explanation), and the outside seventh, G-F, is minor, so G-B-D-F (G7) is a M-m 7th.
Other types of seventh chord include: minor major seventh, minor seventh (minor triad-minor outside seventh), major seventh (major-major), half-diminished seventh (diminished-minor), and diminished seventh (diminished-diminished).