It's covered in great detail here:
http://www.flyertalk.com/forum/hertz...t-you-get.html
From memory though it seems to work out as:
A: ECAR - Economy - Nominally a Kia Rio, which is actually nicer than many in
B: CCAR - Compact - Anything from a Chevy Spark 1.2 to a VW Golf 2.5;
C: ICAR - Intermediate - Corolla/Cruze/Sentra etc.;
D: SCAR - Standard: Theoretically a two-door Altima, could also be a Mustang or some four door cars (e.g., Galant, pre-2012 Fusion);
F: FDAR - Full size - "Midsize" sedans (that may be EPA full-size) - think Malibu/Fusion etc.;
G: PDAR - Premium - Anything larger than that (e.g., 2014 Impala, Taurus, etc.) though a Dodge Charger or 2013 Impala/2014 Impala Limited will be F;
H: LDAR - Luxury - Hertz seems to be moving away from this categorization but includes the Toyota Avalon, Chrysler 300 etc. Many of these would be PDARs with other agencies.