Transmission of information-bearing electromagnetic signal thru a physical medium that separates the transmitter from the receiver.
2 categories: conducted (guided, wired)[exs: twisted pair, coaxial cable, optical fiber] or radiated (unguided, wireless)[exs:radiowaves, microwaves, laser, flashing mirror]. Note: smoke signals, drumbeats, two cans on a string are not electromagnetic.
Or 3 categories: wire or radio or optical.

Limitations: capacity (data rate / bandwidth) and distance.
degradations/impairments cause bit errors:
1. noise (random unpredictable background, i.e. static), thermal, impulse, crosstalk.
EMI (electro-magnetic interference) from fluorescent lighting, electric motors, electric power wires etc. Don't place cables next to.
2. distortion (undesired change in waveform of signal caused by uneven propogation of frequencies in guided medium),
3. attenuation (reduction in power due mostly to dissipation (conversion to heat)) measured in dB, a logarithmic scale: 3 dB is half loss, 10dB is .1, 20dB is .01, 30dB is .001 etc. 0dB as power is 1mW.
Cat 5e limit of 24dB attenuation. i.e. ~99.5% attenuated, only .5% received.
The higher the frequency the more the attenuation. attenuation of guided media
Modulations, power and amplifiers/repeaters, equalization and encodings can mitigate noise and distortion.

Conducted: copper wires (twisted pair and coaxial cable) and optical fibers.
advantages: reliable
disadvantages: longhaul rights-of-way and digging etc, damage (by backhoe, rodent, staple, heel etc): crush, bend, cut.

Radiated: microwaves thru space (not "air") radiated off antenna and received on another.
advantages: no wires-->reconfigurable, WiFi easy installation, but satellite expensive. Mobility.
disadvantages: line-of-sight, weather/environment dependant and variable, limited/regulated EM spectrum