The letter M is derived from the PhoenicianMem, via the GreekMu (Μ, μ). Semitic Mem probably originally pictured water. It is thought that Semitic people working in Egypt c. 2000 BC borrowed a hieroglyph for "water" that was first used for an alveolar nasal (/n/), because of the Egyptian word for water, n-t. This same symbol became used for /m/ in Semitic, because the word for water began with that sound.
Use
The letter 'm' represents the bilabial consonant sound, [m], in the orthography of Latin, as well as in many modern languages. In English, the Oxford English Dictionary (first edition) says that 'm' is sometimes a vowel in words like spasm and in the suffix-ism. In modern terminology, this would be described as a syllabic consonant — IPA [m̩].
^"M" Oxford English Dictionary, 2nd edition (1989); Merriam-Webster's Third New International Dictionary of the English Language, Unabridged (1993); "em," op. cit.