This is Omron quadrature incremental rotary encoder E6B2-CW6C with 1000 P/R (pulse per revolution). The encoder outputs gray code which you can interpret using a microcontroller or Arduino and find out which direction the shaft is turning and by how much. 

This allows you to add feedback to motor control systems. Encoders of this kind are often used in CNC machines, balancing robots and dead reckoning navigation.

This encoder is of improved reliability with reverse connection and load short-circuit protection 

Features

• Resolution: 600 Pulse/Rotation
• Encoding Method: Incremental
• Input Voltage: 5 - 24VDC
• Maximum Rotating Speed: 6000rpm
• Allowable Radial Load: 30N
• Allowable Axial Load: 20N
• Cable Length: 50cm
• Shaft Diameter: 6mm

Datasheet for encoder E6B2-CW6C

Encoder manual and Arduino example code