Cable TV uses the same channel multiplexing as aerial broadcasting, e.g each channel is given its own frequency range or bandwidth, which usually is referred to as FDMA.
A TV signal does not contain one single frequency, actually a fixed unmodulated carrier occupies 0 Hz bandwidth. It is when it is modulated it occupies bandwidth proportial to the modulation rate (symbol rate or baudrate) and the modulation shape/waveform.
A digital bitstream of ones and zeros generates a wideband signal which is not suitable for use in an environment where each communication channel has to be constrained within a given frequency band. Digital communication systems utilises more frequency conservative modulation schemes such as QAM or OFDM/DMT (the later is actually multiple QAM signals in parallell).
But in a twisted pair network cable, only one transmitter/receiver pair utilises each channel and full duplex is achieved by using one pair in each direction. So there is no major restriction on the bandwidth used by the modulation scheme, other than the bandwidth of the cable itself. Up to 100 Mbit/s is quite straightforward but Gigabit Ethernet requires more advanced modulation since the TP cable distorts the signal too much if a raw wideband (several 100 MHz) signal is transmitted. Digital uncompressed HDTV (1.5 GBit) used to be transmitted directly over coaxial cable for a shorter distance of up to 100 ft, but that would not work on TP cables. HDMI transmits the raw bitstream on TP over a few metres.
Apart from that, TP cables suffer from crosstalk and that is a limiting factor in high speed communication on the telephone local loop used by ADSL/VDSL2.
So an ethernet network card for twisted pair do not use a "tuner" to receive the signal such as the tuner in a TV. It uses a receiver that can "lock" to the baudrate/symbol rate of the transmitter. That is why the Ethernet frame consists of a preamble with a repeated pattern that the receiver can use to lock and adjust its receiver clock on (And also estimate and compensate for channel distortion introduced when using long cables).
If several network cards share the same medium, such as with coax networks with mutliple stations or TP cables and a hub, then time division multiple access is utilised using various schemes as described by the other answers.