Physical Differences:

The USB ports for USB 2.0 and USB 3.0 also differ visually.
USB 2.0 has a black or white “block” inside the USB port.
Conversely, USB 3.0 has a blue “block” inside the USB port.
The more recent USB 3.1 port also differs visually in that the block inside the USB 3.1 port is red in color.

Data Transfer Rate:

USB 2.0 has a data transfer rate of 480 megabits per second (mbps).

USB 3.0 has a data transfer rate of 4,800 mbps. This means that a USB 3.0 is approximately 10 times faster than USB 2.0.

More recently, a USB 3.1 has also been released. A USB 3.1 has a data transfer rate of 10,000 mbps. This is twice as fast as USB 3.0 and twenty times as fast as a USB 2.0.

Backward Compatibility:

USB 3.0 ports are completely backward compatible; this means that when a USB 2.0 drive (or earlier versions) is connected to a USB 3.0 port, the drive will work normally.

It is important to note that a USB 3.0 drive is also compatible with a USB 2.0 port. However, a USB 3.0 drive will exhibit the same transfer rate as a USB 2.0 drive when connected to a USB 2.0 port. In other words, a USB 3.0 drive must be connected with a USB 3.0 port to be able to achieve the high data transfer rates USB 3.0 are known for.

Power Usage:

USB 2.0 are capable of providing up to 500 mA current

USB 3.0 are capable of providing up to 900 mA current

USB 3.0 are capable of delivering more power when the need arises and conserve power when the USB drive is connected but not being used.

Larger Bandwidth:

USB 2.0 offer a one-way communication path. This means that data is both sent and received over the same pathway. Thus, USB 2.0 can only send data or receive data at a given time, but not do both.

USB 3.0 uses two separate unidirectional data paths, each with a dedicated function: one for sending data and the other for receiving data. Thus, USB 3.0 can simultaneously send and receive data.

Larger Number of Wires:

USB 2.0 has a total of 4 connector wires.

USB 3.0 has a total of 9 connector wires.

It is the presence of these 5 additional wires which increase the bandwidth of USB 3.0 by permitting two way communications, simultaneously.

The superior performance of USB 3.0 compared to USB 2.0 has caused an increase in popularity for the USB 3.0 USB drives.