The Unicode Consortium has categorized the world's emojis. Every emoji idea has unique five character alphanumeric code. See the full list here.
The standard smiley face emoji appears slightly different across platforms: Apple, Google, Facebook, Windows, Twitter, JoyPixels (an open source emoji designer); Samsung; and Gmail. Softbank, DoCoMo and KDDI are Japanese mobile phone providers. All are assigned the unicode 1F600. A slightly different version smiley face emoji with more expressive eyes receives a different unicode.
Emojis that can have quite significant evidentiary meaning can appear very different in different platforms. A variety of images are used for phones:
Standardizing emojis by unicode allows targeted searches to be run for particularly provocative emojis.
Emojis with the same unicode can convey subtly different meanings depending on which platform's emoji library they come from.