Markus,
FETs can be VERY fast devices. They can easily become oscillators if any 
inductance (nanohenries...) appears in the source lead, including the 
inductance of the bonding wire and pin.

The oscillating L-C circuit consists of the gate-to-drain capacitance 
and the source inductance. Frequency may be in the tens or hundreds of 
megahertz. The oscillation may or may not occur, depending on the Q of 
the circuit. Lossy circuits don't oscillate. Less lossy circuits may do 
damped ringing on turnon and turnoff.

The 100 ohm resistor damps the oscillation. It's a VERY good idea to 
include it in any FET circuit that switches current on and off. For very 
fast switching, such as switching regulators, the resistor may have to 
be tuned to compromise between switching speed and the tendancy to ring.

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