Will someone explain to me in simple to understand terms - what is the difference in some of the newer handheld gadgets I hear about?

Treo?
Hand-held PC?
Pocket PC?
BlackBerry?
PDA?
PalmPilot
SmartPhone?
Sidekick?
IPod?
MP3 Player?
Some play music, some are phones, computers, internet machines, word processors, etc. Some are more than one thing right? Some are the same but brands? Are the computer ones wireless or are they a cell service?