A C shape neck is the most common probably... It's a shallow roundback neck. It's fairly modern in comparison to a U shape neck in that it has shallower shoulders. The shoulder of the neck is the sides. A shallow shoulder means that the angle is smaller. A deep shoulder or big shoulder means that the sides of the neck are generally deeper. Hence the U compared to a C. The flat profile necks are thinner than the C shapes and more modern. They play extremely fast but most older people with backgrounds in blues find them harder to play because they're so thin. Besides just these shapes you might hear of a V profile neck which is also a vintage neck profile that Fender used back in the day. It's shaped just like it sounds... like a V... Also they made a slight V shape neck which was also just as it sounds... slightly V'd. Not as sharp as the standard V. Then there are the Fender Custom Shop profiles like the Clapton and the SRV... The Claptons is a Slight V that is offset towards the top of the fretboard so your hand fits nicely on the neck... it plays like butter. The SRV is a combination neck profile between a C and a V... since his Number One was played so much he actually wore the V into a C at the upper fret areas... from 7th fret up. His Custom Shop Guitar comes with this combination neck profile. Hope that sheds some light on your questions.