Hall effect sensors can sense when a magnet gets near them.
Output goes high when the magnet is removed.
Latching sensors' output goes low when a magnetig south pole is near the sensor, and stays low until a magnetic north pole is near. Linear sensors' output is proportional to the magnetic flux density. Unipolar or latching sensors can be used to sense the speed of a rotating part like a wheel or gear by gluing a magnet onto the wheel and having a sensor nearby. Everytime the magnet passes the sensor, you will get a pulse out that can be read by other circuits. Also, any information can i find in the internet for learning it.