In general, they have 3 digit codes. The first is for the series and the last 2 for the nominal engine size. e.g 318i, is a 3 series, 1.8l injected motor (hence the i). Now the motor size does not work for some of the E36 & E46 exactly at least, as the 323 can have a 2.5l motor. The e suffix is the economy motor. iS is the Sport, LE is Limited Edition, iL is for the limo version. CSL, Coupe Sports Light. M is the most important letter in the BMW dictionary.
This doesn't work for 2002 models and the older stuff either.