What do the ratings actually mean? How do different nations' ratings compare?

Included so far are the UK, USA and Australian rating systems; whenever a film is reviewed and it's main rating is from another country, that system will be added.
See this Wikipedia page for further details.

U	Universal: suitable for people of any age
G	General [Audiences]: suitable for people of any age
PG	Parental Guidance (Suggested): may contain material unsuitable for young children
PG13	Parental Guidance Recommended: may contain material unsuitable for those under 13
12A	12 or Accompanied: under 12s need accompanying adults, may contain material unsuitable for those under 12
12	12: not suitable for those under 12
15	15: not suitable for those under 15
M	Mature: not recommended for those under 15
MA15+	Restricted: under 17s need accompanying adults, may contain material unsuitable for those younger than 17, adults only
R	Restricted: under 17s need accompanying adults, may contain material unsuitable for those under 17, adults only
18	​18: not suitable for those under 18
A	Adults Only: not suitable for those under 18
NC17	​Adults Only: not suitable for those 17 or younger
X18+	Restricted: only available for adults, contains explicit sexual acts
unrated	Unrated, Not Rated: not submitted for rating or including scenes extra to the version that was submitted for rating