Thermistors are special resistors which change their resistance depending on the temperature they are exposed to. These specific thermistors are usually used in 3D printers and corresponds to the #14 thermistor definition in Repetier firmware.
100k means that at 25°C, this resistor will have 100kΩ resistance.
The Beta of 3950 is simply the curve that best describes this thermistor.
This thermistor is attached to a brass M3 screw, which allows one to easily attach it to the 3D printer hotend. These thermistors are rated to about 260°C.
The thermistor is connected to 1m of wires, the area close to the hot end have the wires protected against heat.
in M3 Screw