The 1st number represents the series number followed by 2 numbers which represents the displacement of the engine. Following the numbers are 1 or 2 letters which represent the following:
- i = fuel-injection - d = diesel engine - x = xDrive sytem - L = long wheelbase - C = coupe - c = convertible - T = touring/wagon - t = hatchback - s = sport - h = hydrogen engine - g = compressed natural gas
There are models that begin with the letter 'M' followed by the series number. These are series models that have been tuned for more performance by BMW's M division.
Recently, BMW has been changing their naming convention. For example they have started to put on the side of the cars 'sDrive' or 'xDrive' monikers. sDrive is your stand rear-wheel drive cars while xDrive is all-wheel drive.
I do not have confirmation from BMW but I think that the i in BMW 328i stands for injected. This is just what I have been told by someone who owns one of them. The American BMW site doesn't say one way or another.