Nikon D300 Complete Manual page 75

• used in very warm environments. Those pesky electrons
follow the laws of thermodynamics: they get more willing
to move about as they are exposed to more energy (heat).
The rule of thumb is that every 20°C temperature decrease
results in an order of magnitude decrease in unwanted
thermal electron accumulation
• you set higher ISO values. Higher ISO values generate
more noise because they're obtained by essentially
amplifying the underlying data values, so small disparities
become more visible as you increase the amplification.
Moreover, Nikon DSLRs adjust the ADC circuitry at higher
ISO levels to better match the available bits against the
actual number of photons converted to electrons, and this
has an influence on how noise looks in high ISO shots.
Noise shows up in photos as incorrect pixel values, and is
easiest to see in large areas of a single color (like the sky, or
the background in the basketball image) or in deep shadow
areas where very few photons were collected. Noise
essentially shows up as false detail (detail that wasn't actually
in the scene).
"Dark current" is the name for a form of thermally-induced
current that the sensor produces even when it isn't struck by
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light (thus the "dark" in the name). Each individual sensor
tends to have a different dark current noise pattern, much like
humans have unique fingerprints. That pattern will change a
bit over time, and with temperature. Nikon, like all digital
camera makers, masks off light from some photosites at the
edges of the sensor so they can determine what the sensor
thinks is absolute black (read: the average dark current), but
this system isn't foolproof. Thermal properties can be different
at the center of the sensor than they are at the edge, for
example.
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Thermal electron = internally generated. Photo electron = generated by external
light trigger. We don't want the former and do want the latter.
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Actually, struck by photons.
Thom Hogan's Complete Guide to the Nikon D300
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