Optical (Infra-Red) end stops

If the gap is closed then the output becomes low.

The main shortcomming of this end stop is it's sensitivity to sunlight. So if your printer uses these then keep it away from the window / direct sunlight on sunny days.

Magnetic mechanic (reed switch) end stops

These are not used

Magnetic electronic (hall-sensor) end stops

Since there are no mechanical parts in this sensor this will have a longer lifespan. Altough it is not neutral to the environment.

It needs to work together with a little magnet, and that magnet might just pick up a tiny peice of metal, where in turn the machine could bump at. Though it is probably more reliable then the optical end stop

Note:     Not all endstops are build 100% compatible to the above schema and some might result in an odd behaviour with your firmware. End stops look to simple to go wrong, but regarding the make or break contact is used and a pull-up or pull-down resistor might just be that tiny detail that makes the end stops unreliable or prevend them from working alltogether. (some firmware / end stop combinations just behave bad together) Something like firmware x works fine, but when I load another firmware then the end stops no longer work. If this is messing with your machine then try to understand your end stops. The firmware that is looking for a break-switch is in my opinion the most correct written. Still it would be helpful if the firmware (all firmwares) would have an option to set make or break. So that it is less painful to switch between firmwares.

http://reprap.org/wiki/Mechanical_Endstop

Firmware note: Repetier-firmware had a EEPROM_MODE and in some cases this leads some issues that cause some confusion.  

Endstop status

The M119 G-code command gives feedback on the endstop status (If your firmware supports this)  Teacup, Marlin and Repetier have it implemented.