Unicode is now the only widely accepted standard encoding for writing Ancient Greek (as it is for all languages). Full information about the standard and its implications for classicists may be found here (GreekKeys information page) and here (Stoa Consortium). Older font-based forms (WinGreek, SPIonic, LaserGreek, pre-unicode GreekKeys, etc.) are now obsolete, and will inevitably cause problems in the production and dissemination of Greek text. The Unicode standard has a unique identifying ‘code point’ for millions of individual characters, symbols, glyphs, etc. For instance, the code point for ἄ (alpha with smooth breathing and acute accent) is U+7904. Similarly, the number 500 as found in Attic inscriptions,
is in the Unicode system at code point U+65861. This system of coding allows — in contrast to older font-based solutions, which had a limited number of code points and each used them differently — for enduring compatibility between platforms (Windows, Mac, etc.) and between different fonts which comply with the standard. For the production and display of Greek texts, particularly specialized epigraphical texts, two things are needed:
(1) A Unicode-compliant input method, i.e. a way of telling your computer to produce the code U+7904 when you want to type e.g. ἄ (alpha with smooth breathing and acute accent). This will typically involve a keystroke of the key marked ‘a’ on your keyboard, preceded or followed by one or two other keystrokes); a combination of straightforward Greek with highly specialized epigraphical characters and symbols sometimes requires more than one input method (more on this below).
(2) One or more fonts which contain the characters necessary for epigraphical texts, i.e. a font which actually contains a glyph/character looking somewhat like ἄ (alpha with smooth breathing and acute accent) at the code point U+7904, etc. Very few fonts actually contain a character for all currently mapped Unicode code points. Whilst more and more (though certainly not all) Unicode-compliant fonts now have a fairly full set of Greek with diacritics, few fonts include a glyph for the Attic Greek numeral for 500 under U+65861 and this code point is left empty in most fonts. Some highly specialized fonts exist, however, sometimes including in their ‘private use area’ very specific characters which have not (yet) been assigned a standard Unicode code point. Several other fonts fall somewhere in between with respect to coverage (i.e. they include several, but not all, specialized symbols).
Note: (1) and (2) are not the same thing! If you are changing from Latin alphabet to Greek alphabet only by selecting a different font, you are not using a Unicode-compliant solution.