I am acquainted with Exalted's second edition lore, but am about to sign up for a game that uses third edition of the lore as a base. (It uses mechanics of neither, but this isn't a question about mechanics.)

In order to be more efficient at quickly getting up to date about the world we play in, I ask:
What are the key differences in the lore between the editions?
What parts are critical to read up on immediately because the summary of changes is an oversimplifications, parts can be caught up on later, and what parts are largely the same and thus not worth spending time on?