100nF = 0.1uF = 100000pF Ceramic disc parts are chosen because they are inexpensive, and it is all that is needed for this non-critical circuit function. The 104 marking refers to 1, one zero, and four zeros, making 100000pF. You may see similar capacitors marked as 104Z. The Z refers to the properties of the ceramic dielectric in terms of change of capacitance with temperature.
In the circuit, C2 and C3 do various things.
a) Both provide a straightforward path to ground for R.F. signals in L1.
b) C2 provides a smoothing reservoir capacitor for the rectified radio signal coming down from Q1 to feed an audio voltage into L2.
c) C3 lifts the voltage of pin 5 of L1, and hence the base of Q1 at audio frequencies to provide an ongoing source of audio bootstrap base bias.
d) From a "signals and systems" analysis viewpoint, it's a bit crackers, because four interdependent things are going on simultaneously. PSPICE this one.
