Regarding BMW cars, I am trying to find out what the "i" means in
525i, 540i, etc., and what the differences are in features between the
525i, 540i, and 530i models.

sakauf-ga:

BMW follows a standard practice in naming their vehicle models. Ever
since the mid-1970's, they have used 3-digit model designations that
communicate the relative 'series' of the vehicle, the engine
displacement, and any distinguishing engineering feature. Remembering
that this started in the 1970's, when most cars used carbeuretors, it
should be no surprise that the most distinguishing engineering feature
worth noting was whether a car had fuel injection for the engine.
Indeed, the 'i' stands for 'injected'.

This use of the letter 'i' for street-bragging rights first started on
BMW's famous 2000tii model in 1969:

http://www.thedotcommune.com/NK/history.html

Incidentally, the 'ti' portion stood for 'Touring Internationale', to
showcase its racing series heritage (the TI was a famous racing series
of the time).

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The three models you identified, namely 525i, 530i, and 540i, imply
that you are looking at vehicles from the 2003 model year. The reason
I say this is because, for the brand-new-for-2004  series, there is no
540i; instead, it is now the 545i. As well, for the previous
generation (which spanned the model years from 1997 through 2003), the
530i was only available in the 2003 model year. Prior to that year, it
was known as the 528i.

The different model names tell you, at a glance, the engine size in
the car. The 525i has a 2.5 litre engine; the 530i, a 3.0L engine;
and, the 540i a nominally 4 L engine (note that the engine was
actually a 4.4 litre engine, but in a slight departure from their
naming convention, BMW chose to keep the 540i designation).

To see what the various feature differences are in these three models,
please refer to the following page:

http://auto.consumerguide.com/auto/new/reviews/full/index.cfm/id/23155

If you scroll down to the "Standard Equipment" section, you will see
how, as you move from the lowest model, the 525i sedan, to the 540i
(and the M5 beyond that), the engine grows, and the list of standard
equipment also grows. Note that some of the 'standard' equipment on
the higher models, was available as options for the lower models. So,
for used and unsold 2003 model year 5-series vehicles, you will still
need to review the exact equipment specifications of any particular
vehicle to truly know that options it comes with.

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Note that for the new-for-2004 redesign, the feature differences can
be viewed here (again, scroll down to Standard Equipment):

http://auto.consumerguide.com/auto/new/reviews/full/index.cfm/id/37499

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I hope that this information helps!

Regards,

aht-ga
Google Answers Researcher

extremely complete answer, including interesting additional info with
great links for me to find more detail.