USB Flash Drives problem

Hello, would you mind to tell me that "USB Flash Drives "
http://www.kingston.com/flash/DataTravelers_consumer.asp

is very small size, as same as finger ? but that Capacities is very large (1GB,2GB,4GB,8GB...16GB?)...

what is that substance ?

If around 1 year no used. no insert computer...mean no any extra power,
that "USB Flash Drives" that DATA will Total Loss ?

And If buy USB2.0 (4GB) Flash Drive , insert only support USB1.1 computer, is SURE no problem to run or Not Detect my device?

And If computer is PII computer, maybe error: can't over 2GB problem,
I remeber some Harddisk can't over 2GB. but USB Flash Same ?

And That "USB Flash Drive" need Defrag ? As same as Harddisk?

Any suggest!
Thanks!