Although historically used in a slightly different way, in today's modern theory, a tetrachord is a four note scalar pattern within the interval of fourth. There are fundamentally 4 of them:
Lydian - C D E F#
Ionian - C D E F
Dorian - C D Eb F
Phrygian - C Db Eb F
All of the fundamental scales that are used in Western Music are made of combining these tetrachords together. For instance:
Locrian is made of a combination of Phyrgian and Lydian tetrachords separated from each other by the interval of minor 2nd:
C Db Eb F < --- > Gb Ab Bb C
--C Phrygian--| min 2 |----Gb Lydian----
Now, of course you can come up with more authentic tetrachords other than these four and combine them together in any way you want to create many distinct sounding synthetic scales.
C Db Eb F# -- G Ab B C
in which two altered phrygian tetrachords combined together with a minor 2nd interval.
I don't know how you've got the impression of pianists only dealing with such theoretical things Maelstorm, but the truth is, this bit of theory has been greatly favored by several composers from Prokofiev, Bartok, Stravinsky to John Zorn, John Adams and many jazz giants who based their music on modal harmony, like Coltrane, Miles, Coleman, Monk, Sun Ra etc.