[billyhoffman]

uhhh, I think you mean "funds" not "firms." Defining 3 regions and pigeon holing firms is too much of a simplification of VC markets.

Firms raise funds, that can target specific markets/verticals, as well as company size/maturity.

a16z is a firm. They have multiple funds, including a seed fund, surprisingly named "Seed" which competes directly with YC.

http://a16z.com/portfolio/

Some Firms have only 1 fund. Firms manage and guide their funds, and charge a management fee. While managing a fund is a largely personal intensive business, much like consulting, there are some economies of scale which is why many Firms will have multiple funds. This also further protects against risk.

YC and other accelerators are Firms, potentially with multiple funds, but all of their funds are at the low/angel end.