1-From the appearance:
They differ in the number of ports. The dual type has two ports, while the single type has just one.
Dual fiber optical transceivers use the same wavelength on two fibers. It has two distinct channels or ports, TX is used for transmission and RX for reception.
Single fiber optical transceivers use one fiber to transmit and receive data. So it is bidirectional (BIDI) and usually used in pairs. For example: TX1310nm/RX1550nm TX1550nm/RX1310nm.
3-Application in data rate:
Single fiber optical transceivers are normally used for short distance transmission from 100M to 10G and few in 40G/100G; dual fiber optical transceivers has a wide range which chose by most people.