Simple Answer: USB 2.0 gives a faster data transfer speed than the old USB 1.1. There are no problems of compatibility between USB 1.1 ports and USB 2.0 Flash Drives (and vice-versa). 

More Detail: Finalized in 2001, Universal Serial Bus (USB) 2.0 is a complete overhaul of the Universal Serial Bus input/output bus protocol which allows much higher speeds of reading and writing data than the older USB 1.1 standard.