The old letters aren't that unambigious:

S Sonderklasse (280S, 170S), Sport (300S, model "S")
SS Super Sport (model "SS")
SSK Super Sport Kurtz (model "SSK") (kurz=short)
SSKL Super Sport Kurtz Leicht (SSKL) (leicht=light)
SL Sport, Leicht (300SL) Sports car
SLR Sport, Leicht, Rennenwagen (300SLR) Racing car
E Einspritz (190E) Injection
L Lang (300SEL) Longer rear doors, approx. 10 cm
V Vorn (170V) Front motor
H Heck (170H) Rear motor
C Cupé (250C, 280SLC)
K Kompressor (540K), Kübelwagen (170VK), Kurtz (SSK)
D Diesel (180D, 300D)
T Transporter/Touring (230T, 300TD)
G Gelände (230G, 300GD)

And new letters (since 93) are:

C Compact class
E Excecutive class
S Sonderklasse (Special class)
CL
CLK Sport coupée based on C class
SLK Small open sports car
SL Large open sports car
A Mini car
M 4x4 off road

Numbers bssically indicate engine size for the particular model. But there may be variations to the model:

190E would normally have a 1,9 litres engine (if fact 1,8 litres), and the 190E 2,3 would be the same model (chassis type W201) with a 2,3 litres engine.

The sucessor of 190E is called C180 (new numer series). Chassis is then W202.

Another example is 300E 2,3 which is the W124 chassis, where the base model has a 3,0 litres engine, but the actual version has 2,3 litres.

And then there is a whole bunch of other figures and numbers. Simple, right?