Coptic icon of St. Mark, clearly showing examples of lunate sigma from which the Cyrillic Es was derived
History
The Cyrillic letter Es is derived from a variant of the Greek letter Sigma known as lunate sigma (Ϲ ϲ), in use in the Greek-speaking world in early medieval times. It has no connection to the Latin letter C (C c), which is a descendant of the Greek letter Gamma (Γ γ); however, many languages (for different reasons) apply the value of /s/ to the Latin letter C, especially before front vowelse and i (examples being English, French, Portuguese, Latin American Spanish); see hard and soft C. As its name suggests, Es is related to the Latin S.
In the modern Latinized Cyrillic fonts in use today, the Cyrillic letter Es looks exactly like the Latin letter C, being one of six letters in the Cyrillic alphabet that share appearances with Latin alphabet letters but are pronounced differently (or at least differently from the most common pronunciation). This fact has been frequently abused by plagiarism detector circumventors.
Usage
As used in the alphabets of various languages, Es represents the following sounds: