We follow the Python code style (PEP 8).
The most important stuff:
- All strings are surrounded by double quotes: "text"
- classes use CamelCase (except where Blender needs special class names, e.g. operators)
- methods and functions are lowercase, words separated by underscores: update_objects()
- "private" methods/members start with a leading underscore: _internal_update()
- Property descriptions must never end with a dot.
- Enum identifiers should be valid Python identifiers (don't contain spaces, don't start with a number etc.)
For UI texts (buttons, tooltips, panel names) we follow the Blender guidelines: https://wiki.blender.org/index.php/Dev:Doc/Code_Style#UI_Messages
For UI panel classes, follow this naming scheme (section "Naming"): https://wiki.blender.org/index.php/Dev:2.8/Source/Python/UpdatingScripts
See also issue #47: "Rename UI classes to follow Blender's naming conventions"