The first is the color difference.
USB 2.0 connector colors are available in white, black, red, and so on.
The USB 3.0 connector color is blue.
[When buying USB sticks, the case says 2.0 or 3.0, but if you are not sure, you can easily tell by looking at the connector color]
Another factor is speed.
USB 2.0 has a data transfer speed of 480 megabits / s (60MB / s).
USB 3.0 data transfer speeds of up to 5 Gbit / s (625MB / S) are 10 times faster than USB 2.0.
[However, these are the maximum data transfer rates of the USB ports calculated in theory. In fact, you can limit the data transfer speeds of the USB flash drives we use.] Another thing is the price.
In general, USB 3.0 is more expensive than USB 2.0.