In general radio is practical when distances are very small, for example a number of digital assistants communicating around a conference table, or when running cable would be impossible or extremely expensive. There is almost no limit to the distance you can communicate data by radio. The Voyager 1 and 2 satellites are still sending data, and they are out far beyond the orbit of Pluto. In order to communicate data between two computers we must have a common method for encoding the data on the selected physical media. Transmission can be either digital, or analog.
Digital transmission is refered to as baseband, most analog transmission is refered to as broadband.
In digital transmission, we use voltages, or transitions between voltages to define bits. To encode the signal we also need a clock. In computer science this clock does not keep time, but gives a reqular pulse we can use to time short intervals. Part of what the two computers have to agree on is how fast data will be transferred, and therefore, how long each bit will take. The receiving computer extracts the data by sampling or testing the line at about the center of the tenth of a second interval to see whether a 1 or a 0 is being transmitted.