What is a triad? An interval? A unison? A chord? An extended chord?
A chord is made up of three or more notes played together or as an arpeggio (playing the notes of a chord in a specific pattern). The C Major chord has the notes C, E, and G, but can contain the same notes played as an inversion - upside down. For example, a C major chord could contain three occurrences of the notes C, G, and E: C, E, G is root position, E, G, C is first inversion, and G, C, E is second inverion. In general, the notes of the chord come from the notes of its scale.
An interval is the difference between two notes and is one of the building blocks of music. An interval is most often thought of (and measured) in steps and half steps. For example, the distance (interval) between the notes D and E is one step, whereas the interval between D and D# is one half step. Intervals have names such as a melodic minor second (one half step — one piano key next to another) or a melodic major second (one whole step — two piano keys separated by the one key between them). Other names include minor sixth, major seventh and perfect fifth. Two notes in unison have an interval of zero (they are the same note) whereas an octave is separate by twelve keys on the piano.
A triad is simply three notes played together (though not always readily identified as a chord). Triads are based on a particular scale and are named as minor or major (as well as augmented or diminished). As an example, here are the triads based on a C scale: C,E,G – D,F,A – E,G,B – F,A,C – G,B,D – A,C,E – and B,D,E.
The word unison means that each player is playing the exact same melody or series of notes. In a choir for example, everyone would sing the same part. In almost every case, unison also means that all the players or singers are following the same rules in regard to style and dynamics. It’s important to remember that although (singing or playing) an octave sounds very much like a unison, it is not. Using unison in a piece of music often gives the music strength (volume) and emphasis. It also is a great way to define a particular passage or phrase.
An extended chord contains notes of a chord that are extended (added) beyond an octave. Examples of extended chords are ninth, eleventh, and thirteenth chords. In the case of the ninth chord, the ninth note of the chord’s scale is added to the chord. For example, a C ninth chord contains the note D. Examples of extended chords are ninth, eleventh, thirteenth chords. Using a C chord as an example, the C 9th chord contains the notes C, E, G, Bb and D. Beyond this, it also means that certain notes of extended chords need not be played for the chord to continue to “function” as intended.
A suspended (or suspended fourth chord), contains the fourth note of the chord’s scale. For example, the fourth note of the C scale is F. So a C major suspended chord contains the notes C, E, F, and G. Suspended chords are normally used temporarily and sparingly but add a dimension of suspense to music.

One note played alone is termed a“unison.” Two notes played simultaneously (or adjacent to one another) are termed an “interval.” Intervals are named by the distance between them, so there can be intervals of 2nds, 3rds, 4ths, 5^th, etc.  Chords are three or more notes (or pitch classes; strictly speaking, notes are the written form of pitches) played together. But these notes don’t necessarily have to be played simultaneously. Broken chords, or arpeggios, are three or more notes that aren’t played at the same time but closely enough to be heard as a group or whole. Sometimes in rock music the term “power chord” is used, meaning that the 3^rd is left out of a 3 note chord leaving just the root and 5^th.

Chords are most often named based on their number of notes or the type of intervals involved. Chords classified by note number are given names such as trichord (three notes), tetrachord (five notes), and hexachord (six notes). Chords classified by interval are given names such as tertian (third chords), secundal (second chords), and quartal (fourth chords). Sometimes chords are named based on both qualities. Tertian trichords, for example, are chords with three notes, each a third above each other. These type of chords are actually the most common in western music, found frequently in rock and pop.

These chords aren’t the only chords possible, however. There are several specialized chord types that seem to defy strict categorization. Inverted chords are created by turning any root position chord upside down. Seventh chords can be made by adding a fourth note to a triad — a third above the chord’s fifth — which makes the highest note a seventh from the root. Extended chords are those with notes that extend above a seventh, such as a ninth or an eleventh. But it’s important to mention that no extended chord can go above a thirteenth. By that point, the notes included will have already been played somewhere in the chord, taking it back down to an eleventh or thirteenth.

 A “slash chord” is a chord where the root of the chord is not the lowest note, which would include inversions, but also might include non-chordal notes. An example might be C/B.

 A “sus chord” – short for “suspension” – is a chord where the 3^rd of the chord is replaced by the 4^th.

The Nomenclature of Music: Unisons, Intervals, Chords from chordman on Vimeo.