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An unmodified DSLR has a UV/IR cut filter installed, which limits the transmission to the visible spectrum. Some can get IR, with long exposures, but the majority of the IR is blocked from the sensor.
By Full Spectrum DSLR, I mean one that has had a conversion performed, where a clear filter has been installed in-place of the UV-IR cut filter. This basically opens up the DSLR's spectrum to the full sensitivity of the sensor (from roughly 370 - 1000nm). This way, you can use any external filter you wish. There are other types of IR modifications, but this is probably the most flexible.
Feel free to contact me off-line if you'd like more details or info...