Internationalization and localization (commonly abbreviated as i18n
and l10n respectively) are terms used to describe the effort to make
WordPress (and other such projects) available in languages other than
English, for people from different locales, who use different dialects
and local preferences.
__() is used when the message is passed as an argument to another function. _e() is used to write the message directly to the page. More detail on this @ http://codex.wordpress.org/Translating_WordPress