Here are some of the more popular cable types used to transmit bits between computers:

• twisted pair -- a pair of insulated wires twisted around each other, suitable for short distances.
• coaxial cable -- a copper core surrounded by insulated surrounded by a hollow wire tube. This is what is used for cable TV.
• fiber optic -- a version of coaxial cable only the core is an ultrathin strand of fiberglass through which light pulses are passed. This is the fastest and most secure, but also expensive.

Since wires always seem to have either 1s or 0s on them (they either have a high voltage or a low voltage, or no-light or some-light in the case of fiber optics) it is hard to say exactly when a packet starts. Synchronization is needed to enable the sender and receiver to agree on the bit boundaries and they also need to agree on a common bit rate in order to set their clocks accurately. Then, even tick of their internal clock, they sense the communication's wires voltage (or light amplitude) and register either a 1 or a 0. These are often difficult and complicated issues which we can only hint at here.