Modulation is an essential part of digital communications. There are various schemes available and there are design compromises to be made between data rate, bandwidth, the likelihood of errors, complexity and so on. Modulation is used both in wireless and wired communication.
In digital communication, the unit of data transmission is the symbol. A symbol might represent a single bit of data (such as 0 or 1) or several bits (such as 0000, or 0001) depending on how many symbols are used in the modulation scheme.
Quadrature amplitude modulation (QAM) is a very widely used digital modulation system for providing multiple symbols.