What is Morse Code?

Morse Code is a method of transmitting textual information as a series of on-off tones, lights, or clicks that can be directly understood by a skilled listener or observer without special equipment. 

The International Morse Code encodes the Roman alphabet, the Arabic numerals and a small set of punctuation and procedural signals as standardized sequences of short and long signals called "dots" and "dashes" respectively, or "dits" and "dahs". Because many non-English natural languages use more than the 26 Roman letters, extensions to the Morse alphabet exist for those languages.

Each character (letter or numeral) is represented by a unique sequence of dots and dashes. The duration of a dash is three times the duration of a dot. Each dot or dash is followed by a short silence, equal to the dot duration. The dot duration is the basic unit of time measurement in code transmission.

Morse code speed is measured in words per minute (wpm). Characters have differing lengths because they contain differing numbers of dots and dashes. Consequently words also have different lengths in terms of dot duration, even when they contain the same number of characters. For this reason licencing bodies, such as the Federal Communications Commission (FCC) in the US, pick a standard word to measure operator transmission speed. "PARIS" and "CODEX" are two such standard words.

One important feature of Morse code is coding efficiency. The length of each character in Morse is approximately inversely proportional to its frequency of occurrence in English. Thus, the most common letter in English has the shortest code, a single dot.
By Wikipedia