Can you help me make sense of the different cell phone operating systems?

I stopped by a phone store to see what was up over the weekend. I couldn’t believe the number of phones I saw. Eventually, the salesman told me they were grouped by operating system. If I understood him correctly, there are now four different phone operating systems; each requiring a phone designed specifically to use that OS. I.e., you can’t run more than one OS on any phone. I gathered that the four operating systems were: iPhone OS, Sidekick, Blackberry, and Android (a google concoction).

I see where iPhone and Android came from (Android is a google thing), and I’m guessing that maybe Blackberry is a successor to the Palm Pilot? Or is it a totally new thing? What’s Sidekick’s heritage?

They all seem to do the same kinds of things, but maybe that’s not true? What are the differences in what they can do? What are the advantages and disadvantages of each?