Is penicillin a stronger form of amoxicillin?  If not, what is the
difference between the two?

Penicillin is both a name for a group of chemicals and a chemical
itself.

The chemical penicillin is also called benzyl penicillin, or
penicillin G; amoxicillin is slightly different, you can see the
chemical formulas of both here:
http://www.chemsoc.org/exemplarchem/entries/2002/stanley/03b_Modification/Modifi19.gif

Amoxicillin is active against some Gram-positive and some
Gram-negative bacteria, while penicillin only is active against some
Gram-positive bacteria and Gram-negative cocci. (Some bacteria produce
a substance (penicillinase) that ruins penicillins effect.)

I guess you could say Amoxicillin is stroger than penicillin since it
works on a wider variety of bacteria; but doctors usually choose the
antibiotic they think works best on the type of bacteria they want to
kill.

tisme-ga; this is in no way ment as critisism to your answer, just
wanted to clear up some things:) Penicillin is usuallly used as a name
for the whole group of chemicals, benzyl penicillin is the name
commonly used for the chemical also known as penicillin.

:) Arsenic

Reading thru my comment I feel like writing about the Artist Formerly
Known As Prince...

Forgot to name my sources: 

The Merck Index, 12th ed.
http://www.felleskatalogen.no (the Norwegian pharmaceutical codex)

...like I am writing about... Also, sorry for all the lousy English!

Hi Arsenic! Thank you so much for taking the time to post those
comments.  It was exactly what I wanted to know!

Hi julie99,

I am happy you got the answer you were expecting. I have already
requested that the editors remove my answer in fear that my
clarification would only confuse you further.

tisme-ga

Hi tisme-ga:)

I wish there was a way for us non-experts to contact you experts with
comments like this... With penicillin used as a name for both the
group of chemicals and a single substance; they are very easy to mix
up.

:) Arsenic