Solenoid Valve  is a valve used to control the direction of air (or fluid) by energizing an electrical coil, together with a spring return or a second coil when the valve must move to/hold another position.  When current flows through the coil, the magnetic field pulls the plunger/armature to shift the valve; when power is removed, a spring (typical for single-solenoid designs) returns the valve to its normal position.

Solenoid valves are commonly classified by how the valve shifts:
1) Single solenoid (solenoid actuated, spring return/monostable) 
2) Double solenoid (two coils; switches between two states/bistable)

They can also be classified by application, such as water, gas, hydraulic, and pneumatic solenoid valves.

For pneumatic solenoid valves, common configurations include 2/2, 3/2, and 5/2-way, and N/O (Normally Open) or N/C (Normally Closed), where NO/NC describe the de-energized state.  A 2/2-way valve is a simple two-port, two-position on/off valve and can be used for air/gas and also for liquids (e.g., water/oil) when the valve is designed for that medium.