As an addendum to Robert's excellent explanation, the story of
how Tully & Fisher came up with it is pretty fascinating in its own right.
I hope this isn''t off-topic, but one of the key terms mentioned early in the Tully-Fisher article is the 'virial theorem'. Understanding what that means is important because it so often comes up and in so many contexts—galaxies, globulars, cluster formation, the ISM, you name it. I've yet to read an explanation of the word 'virial' and the related terms that go with it—'theorem', 'radius', 'mass', and so on—written by someone whose mother tongue isn't partial differential equations.
Any chance you'd like to take a crack at it, Robert? You have a great ability to turn the complex into the lucid.