IDEO's Tim Brown first wrote about T-shaped people in Strategy By Design. A T-shaped person is essentially someone with a core principal skill (the stem of the T) that is complemented by a range of complementary(!) skills which form the horizontal bar of the T.
Zach Smith wrote about multiple shapes on his The Capacity Evolution Blog. I have summarised them visually below. Click on each one to link to Zach Smith’s write-ups.