See we have
U (universal)
PG (parental guidance)
12 (above 12)
15 (above 15)
18 (above 18)
18 is classed as adult.
M is the rating for Mature.
It applies to movies in Australia and ESRB rated games as far as I know. 18+ stuff.
-=EDIT=- Correction.
M is for mature (18+) for ESRB rated games.
M for films in Australia is basically like 12 or 15 in the UK, or PG13 in the USA.