Maybe it's cheating to lump the telegraph, telephone, radio and television into one "invention," but the development of communication technology has been a continuum of increased utility and flexibility since Samuel Morse invented the electric telegraph in 1836 (building on the prior work of others, of course). The telephone simply refined the idea by allowing actual voice communications to be sent over copper wires, instead of just beeps that spelled out the plain text in Morse code. These communication methods were point-to-point, and required an extensive infrastructure of wires to function.
Transmitting signals wirelessly using electromagnetic waves was a concept worked on by many inventors around the world, but Guglielmo Marconi and Nikola Tesla popularized it in the early 20th century. Eventually, sound could be transmitted wirelessly, while engineers gradually perfected the transmission of images. Radio and television were new landmarks in communications because they allowed a single broadcaster to send messages to thousands or even millions of recipients as long as they were equipped with receivers.
These developments in communications technology effectively shrank the world. In the span of about 120 years, we went from a world where it might take weeks to hear news from across the country to one where we can watch events occurring on the other side of the globe as they happen. The advent of mass communications put more information within our grasp and altered how we interact with each other.