What does 2.0 and 3.0 mean on a flash drive?

Speed — USB 2.0 offers a transfer rate of about 480 Mbps, whereas USB 3.0 offers a transfer rate of about 4,800 Mbps which is equal to about 5 GB. This super-fast transfer speed of USB 3.0 can be especially useful when backing up or transferring large amounts of data, such as an external hard drive.