Locales are definition files that tell a computer about the cultural conventions of a language and/or country. So a locale will define how characters are sorted, how the date and time are represented, the names of the days of the week and months of the year.
These are the locale files that interest a localiser:
To help with defining locale data you will also need to know about:
The CLDR is a unification of the various locale information that uses the locale data markup language specification. The CLDR is an important project to contribute your locale data to as this will eventually become the authoritative repository for locale information.