This sensor is essentially a reed switch, encased in an ABS plastic shell. Normally the reed is 'open' (no connection between the two wires). The other half is a magnet. When the magnet is less than 15mm away, the reed switch closes.

They're often used to detect when a door or drawer is open, which is why they have mounting tabs and screws. You can also pick up some double-sided foam tape from a hardware store to mount these, that works well without needing screws.

Specification: 

• Model : MC 38
• Contact Capacity : 1A(Maximum Switch Current)
• Voltage : 12V
• Rated Power : 12W
• Actuation Distance : 15mm
• Size(Approx) : 30*15*28 mm
• Color : White
• Housing Material : Plastic

Examples:

Adafruit IO to Make an IoT Door Detector

 Mystery Box: The Nautical Crate