copied this from another forum:
"International Standard ISO 5775-2:1996, Bicycle tyres and rims — Part 2: Rims: document:
"ISO 5775-2 defines designations for bicycle rims. It distinguishes between
Straight-side (SS) rims
Crotchet-type (C) rims
Hooked-bead (HB) rims
Rims are designated by their nominal rim diameter and their nominal width, separated by a cross (× ). Both are measured in millimeters. The rim type codes SS or HB precede the rim designation, whereas code C is appended to the nominal width."
some examples would be: SS 400×20, HB 422, 700x23c, the "C" designating a Crotchet rim.
That's a funny and incorrect explanation for two reasons:
1 - crochet and hooked bead are the same thing, just depends on whether your talking French or English.
That quote comes from the Wikipedia article about the ISO standard and lacks sourcing. The actual standard is behind a pay wall, so I can't confirm whether ISO actually has differentiated between Crochet and Hooked Bead, which would be very strange.
http://www.sheldonbrown.com/gloss_cn-z.html#crochet
2 - 700C is a french size. The ISO match is "622." Explaining the C by referencing ISO makes no sense. (The French system defined the outside tire diameter. 700a was a larger rim intended to fit a narrower tire, resulting in a 700mm outside diameter, while 700c was intended for wider tires, still resulting in the same outside diameter)
http://www.sheldonbrown.com/tire-sizing.html