Esperanto is the most popular of various artificial languages, invented in 1887 by L.L. Zamenhof. For good info, check out the Wikipedia article.
Esperanto uses essentially the same alphabet as English, but with the extra letters ĉ, ĝ, ĥ, ĵ, ŝ and ŭ. To type these in OS X, you can activate the the US Extended keyboard layout in System Preferences/International/Input menu. The letters with the ^ (circumflex) over them can be typed by doing Option + 6 followed by the letter itself. The ŭ (u-breve) is made by doing Option + b followed by u.
You can also download an Esperanto keyboard layout from my iDisk, which will let you type the special letters more easily (the accented characters are at Option + the base character).