How did Samuel Morse's telegraph work?

The old fashioned Morse telegraph system used wires similar to telephone lines but did not use speech. Messages were sent from one place to another by using a series of short and long electrical on/off signals - called dots and dashes - in unique patterns which Morse invented to represent letters, numbers, spaces and other characters. This coding system soon became known as "The Morse Code".

Another answer
A Morse telegraph works by someone sending a series of long and short electric pulses through a long wire to another telegraph device by using using a version of the "alphabet" called Morse code. The receiving station interprets the pulses into a series of clicking sounds and/or marks on a strip of paper. By using the same Morse code the person at the receiving end of the telegraph can understand the message by reading how long the lengths of the clicks or marks are. The telegraph system was the first step to the development of the telephone.