GitHub uses emoji shortcodes for emoji insertion which replace the code with the native emoji character after entering.

Eg: typing :heart_eyes: replaces this string with the 😍 Smiling Face With Heart-Eyes emoji. Some emojis have multiple shortcodes on Github, such as 💩 Pile of Poo which can be inserted using :poop: :shit: or :hankey: interchangably.

Emoji shortcodes are not standardized across platforms. For example, codes from Github sometimes differ from those used on Slack.

Codes exist for emotes (non-Unicode emoji characters) which are inserted using image-replacement characters. A number of these use graphics of Doomguy from the 1993 video game Doom.

Custom emojis / emotes supported on GitHub are::

:bowtie:
:neckbeard:
:octocat:
:shipit:
:trollface:
:fishsticks:
:suspect:
:hurtrealbad:
:feelsgood:
:goberserk:
:finnadie:
:rage1:
:rage2:
:rage3:
:rage4:
:godmode: