Sunbird is a standalone calendar application, and Lightning is a calendar extension for Mozilla Thunderbird.
New features of Mozilla Sunbird 0.3 include new storage architecture, more intuitive interface, redesigned preferences, add-on support, better printing (a new feature for Mac users), 24-hour views and more. Lightning 0.3 includes 24-hour views, reliable alarms, improved localisation support and more.
Lightning 0.3 is the latest release of calendar extension for Mozilla Thunderbird. Users who wish to use calendar as a standalone product, please choose Mozilla Sunbird.
For more information, please refer to Sunbird and Lightning website. Sunbird 0.3 Release Notes, and Lightning 0.3 Release Notes.