This page gives an overview of the model suffixes that indicate variants of the main model.
p: Professional. Usually with higher specs, bigger/higher density screens, designed to be desktop replacements.
s: Slim. Usually low-voltage, low frequency, soldered-on CPUs, better battery life. Slimmer build, usually thinner than regular models.
i: Budget models, usually but not always weaker CPU (often Celeron), lower resolution, no UltraBay, lacking features (Wifi, Firewire etc.)
e: Economy(?). Similar to i models in weaker specs and features.
u: Ultraportable. Only present on the T430u model (?).
Suffixes on older models vary considerably and do not follow a regular pattern.
Examples are XD, XL, C, CE, CV, CS, CX, CSD, CSE.