In Settings -> Accounts -> Google, when I tap on my Google account, I see a "Sync settings" page with a list of different Google services that can be synced (App Data, Calendar, Drive, Gmail, Google Play Books, etc.).
This list includes both "Contacts" and "People details." What is meant by "People details" and how does this differ from "Contacts?"
Are there any negative outcomes I would expect from disabling sync of "People details" (i.e. what would break?)
This is a little bit of a guess since you didn't specify your device... Contacts is normally part of the stock Android operating system while People is usually part of the launcher or service provider software. They are virtually the same app with different skins or interfaces. The People apps usually also incorporate all of your contacts, including social media. Contacts will most likely only show Gmail and phone contacts unless you alter the settings.