USB 1.0 was launched in 1996 and had a sped of 1.5 Mbit/s (Low-Speed) and 12 Mbit/s (Full-Speed) meaning it would take over 10 minutes to transfer 1 Gbyte of data.
USB 2.0 was released in 2000 with a much higher speed of 480 Mbit/s allowing us to transfer 1 Gbyte of data in around 17 seconds.
USB3.0 will ramp things up again and increase the maximum speed to 5.0 Gb/s allowing us to transfer 1Gbyte in less than 3 seconds, 25GB of data will be transferred in around 70 seconds. That is pretty impressive speeds, I could fill up a 1 terra byte hard drive quite easily in my lunch break!
It is worth noting that we won't be seeing any USB 3.0 thumb drives for quite some time. Products aren't due until the end of 2009 and because it has taken so long to finalise companies are dragging their feet when it comes to implementing it. Microsoft itself has told developers to hold fire until the technology has proven itself, and has said that it will not be including support for the technology in Windows 7 when it ships.