The first circuit in digital recording is the "anti-aliasing" low-pass filter which removes any incoming frequencies above half the sampling rate(22.05 KHz for a 44.1KHz CD sampling rate). The reason is that at least two digital samples of a frequency are necessary in order to reproduce it perfectly; above half the sampling rate only one sample could be taken. This one sample wouldn't define the specific frequency sampled, but would allow several different "alias" frequencies, as John described, to intrude lower down in the audible range.
Then, the last circuit in the process is the "anti-imaging" filter which removes frequencies above the audible range which were created as artifacts during the sampling process and which when removed leave the original perfect analog waveform which entered the system.
