The keyboard is designed for people who use the regular U.S. keyboard heavily, but occasionally need to type other Latin letters (especially accented ones), symbols, and punctuation. In particular, the keyboard supports most of the Windows-1252 (U.S. and Western Europe) repertoire, as well as almost every Latin letter in Unicode.
This keyboard handles only the extended Latin alphabet. If you want a Greek, Russian Cyrillic, or full IPA keyboard, I recommend the standard Microsoft Greek, keyboard, the Russian Phonetic YaWert keyboard, and the Benct X-Sampa keyboard respectively.
There are two basic ways to type characters other than the regular ASCII set. A few characters are directly typed by holding down AltGr and pressing another key. For example, to type the character æ, simply type AltGr+a. As you might expect, the capital version Æ is typed as AltGr+Shift+a. However, the great majority of characters are typed using AltGr plus some key, followed by another key that doesn't use AltGr. For example, the letter a with acute (á) is typed with AltGr+' (that is, AltGr plus apostrophe) followed by a, or by A if you want the capital a with acute (Á).
Combinations like AltGr+' are known as "dead keys", because they appear to be dead when you type them; you need to press a following key to actually input a character. The Moby Latin keyboard has a total of 21 dead keys: 16 for specific accent marks and modifying strokes, one for curly quotation marks and other punctuation, one for math symbols, one for arrows, one for pointing hands, and one for the IPA letters needed for English. (The math symbols were taken from the space cadet keyboard, though Moby Latin is not a full superset of it.) Some of these dead keys are typed using AltGr+Shift, which makes them a little awkward to type, but they are intended to be as easy to remember as possible.
This keyboard and the associated documentation are Open Source, and may be freely copied and modified. The license terms for both is the MIT License. Use, share, and enjoy!